10 years is a long time to wait for an answer, I have two of these great little rifles in my collection, one has the same markings as yours but is marked "KAL.22 Lang FUR BUCHSEN" but has no mention of 5.4mm it also has a four digit serial number, it is also undated.
The second rifle is marked "ROECHLINGSTAHL" on the side of the barrel and "Duetches Sportmodell" on top of the barrel, the crown over B & U are I think Germanicon proof marks. The receiver is marked "Waffentadt" over a crest with "Suhl" underneath, it also has a 4 digit serial number and the calibre is marked as 5.4mm, this rifle however is dated at 1935. I have checked the books that I have and I can't find any reference to manufacturing dates other than saying that these rifles were made prewar. As for finding a replacement stock I think you might have to wait a lot longer tan 10 years before one turns up...