Here's the pic from the URL of a Swedish site selling M40 stocks that Olle mentioned...for 1250 Kronor!!!
That's outrageous!!!

I'm sure I've seen them on eBay for 600 - 700 Kronor!!!
Anyway...Interesting method of attachment...Two straight-cut clamps with a center push-bolt...Slide it on and then tighten the push-bolt against the gripframe lug...
I think I could possibly make up something like this attaching piece...But I like the Luger curved ironwork with the half-moon latch...
I like the shape of the Luger board stock better too...
If I were to attempt such a board stock, I'm thinking of a Luger artillery board stock with a Lahti attaching piece, and a Luger artillery repro holster attached to the board stock, probably with three or four 1/4" carriage bolts holding the leather to the wood...Maybe with domed nuts on the carriage bolts...