Herring Gull H - 1172372cy (argenteus), April 19 2002, Boulogne-sur-Mer, France (50.42N,1.34E).

A bird from Belgium: ringed Bruxelles H-117237, ringed as pullus on July 09 2001 at Zeebrugge (51.20N 03.11E). In general, very similar to this Dutch 2cy argenteus. All the coverts are still juvenile, while eight upper scapulars are moulted to third generation feathers, with a pattern similar to second generation feathers. These third generation scapulars can be recognised by the buffish tone. Most other scapulars are worn second generation, except the rear lowest lower scapulars, which are still juvenile.

Argenteus will show a second partial moult of head and body-feathers in spring (after the previous post-juvenile moult). This partial moult starts in January and ends by May, bringing birds in so-called "first summer" plumage. The head and under-parts will turn white by June.
The juvenile tertials, lesser, lower lesser and median coverts have a notched pattern, worn by April. The juvenile greater coverts show a 'piano-key' pattern; in argenteus on the outer greater coverts as well. This pattern has gone lost as the white parts of the feathers are worn away. The juvenile tertials are very abraded as well. 
The secondaries, primaries and primary coverts are bleached, but the juvenile inner primaries still show an obvious pale window, prominent from below and above. The under-wing is rather uniform grey-brown patterned. The tail has a broad sub-terminal band and isolated dark bars on the basal half.
By April, some argenteus will show three generation of scapulars: still a few juvenile scapulars in the rear lowest row (first generation), a few very fresh lower upper scapulars with a clear buffish tone and a neat white fringe, still with a similar pattern as the second generation feathers (third generation) and most scapulars are slightly worn upper scapulars and upper lower scapulars (second generation). In this photo you find a clear example of this moult sequence.