Just for reference: comparison of underwing primary patterns in Dutch adult Herring Gulls and Lesser Black-backed Gulls.
All wings are of different individuals.
Images were taken during ringing activities in the IJmuiden Forteiland gull colony, the Netherlands, May – June 2018.
Note especially the huge variation in the tongue shape on P10 in Herring Gull, thereby making it a pretty useless identification feature.
Click for a larger view.
For images of upper wing primary patterns, see: