For what I can see you have a k98, mixed , sanded German stock. You're correct, on the barrel channel there is the manufacturer/date of production stamp; maybe my eyes, but it seems to me that on the right side of the buttstock there are some remnants of the WaA's (should be three). Judging by the takedown disc, I bet this is not a Mauser made stock.
I was interested that you typed date............... and no WAa markings !!! why i no idea but im ok with that.
If the ';40' is the manufacturing date, it would tally with the flat buttplate. This looks like a nice, laminated beechwood stock ; later in the war most of these were made with cupped buttplates to guard against cracking. The red glue is characteristic and quite attractive - the gunstock should clean up nicely.
Yes, 40 is the year, 29 the week and "bu 7" should be the maker's code and batch number