Not only is it a bit of nostalgia but from Clare College, Cambridge some light on the subject. Evidently the expression 'stockamsel' was first coined by Johann Andreas Naumann he of (naumanni) Lesser Kestrel and Naumann'sThrush fame. It relates to young males resembling well marked females. This is a dark billed first year male with white fringed breast feathers creating a clearly visible 'scaly' effect. Ageing as a first year would be easy anyway - note the brown 'juvenile' feathers in the wing and the odd one in the body (e.g. just behind the ear coverts).
We also ringed quite a few newly arrived Goldcrests, a female Blackcap and this rather lovely Treecreeper (note the long stiff pointy tail which they use like a third limb pushed against the tree as they work the bark).
