I purchased this rosewood 20 key via Ebay last week, it was advertised as a "Louis Lachenal", the label in the cartouche is a modern copy of the correct label, the serial number is 12112 which if it is a genuine Louis would date it by my reckoning to about 1863, it has the original case which is very well made of solid timber. There is some conjecture as to whether it is a Jones or a Lachenal, so here I am asking for those with much greater experience than mine to advise their opinion.
The bone keys were originally all numbered, but numbers on the most keys have been erased by wear (up to 2mm), at some stage a wooden extension has been added to the air button; the bellows look original but need significant repairs if not replacement, but are good enough to establish all notes work and are in tune (A440) so at some stage it has been retuned, it plays quite well.
Comparing it to my 30 key Lachenal S/No.164895, the dimensions are the same, the fretwork is different, the bellows papers are the same, the keys are a different size 6.4 mm on the 20, 5.8 mm on the 30, spacing is the same. The fretwork on the 20 looks identical to that on the Louis Lachenal in the Concertina Museum #C290. The levers on both are the same, flattened round brass, hooked not riveted, the levers on the 20 are under staples, whereas on the 30 they are through rectangular flat steel posts. The overall construction of the 20 key looks superior to that of the 30, the routing is cleaner and more precise, the reed shoes fit perfectly, each is very clearly stamped with it's note, the stamping on the 30 is very rough and not distinct. The brass reed shoes are 1.91mm thick, reeds are steel. Internally the only marking is the serial number, externally the palm supports lack the trade mark and "Steel Reeds" stamp, but from the amount of wear appear original, on the sides of both ends roughly adjacent to the air button is stamped "C.STAFF"