In 2cy nominate fuscus, there is a large variety in moult stage in August and it's hard to describe a 'typical' plumage for this age class. The images right illustrate the variation from slightly delayed moult, to 'average' and well-advanced birds. Especially the latter group bears a strong resemblance to 3cy birds from the western graellsii and intermedius populations in general appearance. So far, this page contains images from a short trip to Stockholm, Sweden and to the landfill of Tampere, Finland in August 2002. There are a few points, which can be found more often in fuscus than in graellsii/intermedius, on population level and may give some clues as to the specific identification in some characteristic birds. Note however that single individuals from graellsii, intermedius and fuscus are almost impossible to tell apart as there exists a large overlap. For comparison, have a look at the images of ringed graellsii and unidentified graellsii / intermedius presented in the Lesser Black-backed Gull sections of August, September, October and November-December and on the page "Discusfus".

Primary moult stage

Prior to northbound migration all or nearly all primaries were moulted to second generation flight feathers on the wintering grounds. From February to May 2cy fuscus moult all (or nearly all) primaries to second generation feathers at the wintering grounds. The complete month of June is used to migrate north. Lars Jonsson, in Birding World (1998 Vol. 11 No. 8) mentions the arrival date in Scandinavia as pretty standard "arriving promptly around 1st of July on Gotland". 
Ringed 2cy fuscus at Tampere in Finland all showed advanced moult in the primaries. Most birds had moulted P1-P9 or P1-P10 on the wintering grounds in spring. Late July and early August, typical 2cy fuscus start shedding the inner primaries for the second time, growing in third generation primaries. Remember that these birds are still only one year old! 
In western Europe, some 3cy graellsii / intermedius may show a low moult score by July and resemble these 2cy fuscus. Best clues in separating the two may be the condition of the primaries: 2cy fuscus normally has brand-new dark, blackish outer primaries, moulted in April-May (3 months ago). In general, August 3cy graellsii - intermedius have the second generation primaries already bleached pale brown and worn at the tips, as the primaries were moulted 8 months earlier. 

LBBG "Dutch intergrade" 3cy, May 31 2002, Maasvlakte, the Netherlands. Note the bleached 2nd generation primaries.

Larus fuscus fuscus 2cy, August 06 2002, Tampere, Finland. Note the fresh 2nd generation primaries with nice tips.

Identification of 2cy fuscus is easier if there are still juvenile outer primaries (arrested primary moult), especially when a new moult wave starts at P1. Juvenile P10 is never present in 3cy graellsii or intermedius.
However, some 2cy fuscus return to Scandinavia with all juvenile remiges. Lars Jonsson, in Birding World writes: "Birds seen in early July have shown a wide range of variation, but the majority have already finished moulting all of their flight feathers, two had obviously suspended their moult at score 45 (p10 unmoulted) prior to migration, while others had obviously returned to the Baltic with their juvenile flight feathers intact (like most other large gulls)". 

These birds may be hard to separated from intermedius, especially when larger males are involved. Moreover, when these birds start moulting the primaries in July and in the mean time develop second generation wing-coverts with barring or anchor patterns, identification of intermedius, fuscus or heuglini may be very difficult (see e.g. image 3598). 
The last image shows a bird very similar to 3cy fuscus present at the same time, especially considering soft part coloration and advanced wing-covert moult. However, the tail is still worn second generation, without (at least some) typical complete white tail-feathers seen in all 3cy fuscus. The outer primaries are still second generation, though slightly bleached brown. These birds may have moulted the complete set of  remiges during early winter and have stayed on the wintering grounds long enough for these feathers to become bleached.

Secondaries and rectrices

On first glance the average 2cy fuscus seems to have moulted the complete tail and all of the secondaries on the wintering grounds. By August the tail-feathers appear already worn at the tips and the brown sub-terminal band might be slightly bleached towards paler brown. This is in line with the statement of Jonsson: "Although it is more difficult to judge in the field how many of the secondaries have been moulted, they generally appear to be largely moulted, but a few juvenile secondaries sometimes remain". The number of observation of fuscus in Finland (in our case) are too low to draw any conclusions, but at least one bird had some inner secondaries still juvenile (the only 2cy bird we caught). The tail-feathers are easily judged in the field and appeared second generation in all 2cy birds.

Wing-covert moult

It's again the kind of birds represented in image 3598 that show most juvenile wing-coverts, especially the upper rows of lesser coverts, the carpal edge (outer lesser coverts) and the central greater coverts, which are moulted in descending order. However, the average fuscus has all wing-coverts moulted on the wintering grounds prior to northbound migration and those second generation feathers appear already pretty bleached with paler brown centres and abrasion on the fringes. Quite some birds were missing the outer median coverts, indicating they started a partial moult again, continuing to replace wing-coverts for the second time and grow in third generation feathers. In general these third generation feathers are all dark, blackish and can be found in almost all the images presented on the right-hand side. Advanced birds show many third generation wing-coverts, as can be seen in images 3323 and 3353: Most of the median coverts, the inner greater coverts, most of the lower lesser coverts and the outer lower lesser coverts have been replaced by third generation coverts. As can be seen in the images, the old second generation wing-coverts are bleached and abraded at the fringes.

Literature

Lars Jonsson (1998), "Baltic Lesser Black-backed Gull Larus fuscus fuscus moult, ageing and identification", in Birding World Volume 11 number 8, 1998.

Visa Rauste (1999), "Kennzeichen und Mauser von "Baltischen Heringsmöwen" Larus [fuscus] fuscus und "Tundramöwen" L. [fuscus] heuglini", in Limicola 13 1999.