Sorry to cause you fun - not! I had guessed that all was known about 1920s Excelsiors, but clearly there is a little way to go.
Now I don't need to know any more, but it was hot today and I made a spreadsheet while I watched the cricket. I think there is some sense emerging...
You're certainly correct about the beginnings of the Excelsior junior/miniature with the 147cc Villiers engine for the 1923 model year. The model was announced in October 1922 - see below, and from the VMCC frame and engine numbers, plus those from the web the frame numbers were, as you say, 4 digits with no prefix, at least on the lightweight. MkVI-C, then VII-C, then VIII-C engines over the next few years, and frame numbers with prefixes K, L, then D over the years (say) 1924, 1925 and 1926?? There are c1926 frames with D prefixes for lightweights (std and ladies'), 300cc JAP, 350 ohv JAP, 550 Blackburne, ao it seems a range-wide code? Of course it's hard to be certain with mixed bits during restorations, etc.
Anyway, great to get a pic of the chain-drive "de luxe" lightweight in Nov 1924, so a 1925 model. Matches the frame in question perfectly. At least the caption to the pic says it's a 1 1/2 h.p., but the carburettor is on the left side of the cylinder. I'd expect it to be out the front on the right for a 147??
Leon