Thank you!!
Like I thought: no takedown disc, hole in the buttplate, LION proof on back of the bolt stem. It is as should be!
The top of the receiver is a bit mysterious, but it seems that it had been scrubbed which is so so obvious now as the rifle clearly shows traces of wear and patina.
I have searched my database and found another Brno Mauser which is 100 percent like this one - in brandnew condition. Scrubbed receiver, "winter trigger guard" and all. And I found something there which was quite amazing: a "Js" and a number, which supports my rack-number-theory.
It seems that lots Jx and Kx are the last reported barrel lot codes in production.
Like heavy_mech said, Jx and Kx are the last known barrel codes assembled during the war. The barrels on the Brno Mausers in my database are from the Lx "series" so it all fits together very well. Near the end of known postwar Brno pieces, there seems to be a switch to "Nx". It also appears that a number of spare barrels in the the "Mx" series have been exported.
All this deserves further research, of course.
Greetings
T