I have quite a few Anschütz rifles.
Two are marked as a "Mark 12"
One is almost exactly as another one marked as a Model 2000, see this link and note the acorn shaped bolt shroud.
http://jga.anschuetz-sport.com/downloads/Ersatzteile/EN/ET_Mark2000_1982-05-01_EN.pdf?downloads
The other is a Junior Achiever rifle that also use the acorn shaped shroud.
These bolts are shorter the the 64 model and are not interchangeable with the 64's(CIL 180 &190's)
The Mark 12 and Model 2000 below.
With Cil 190's
This shows the acorn/wasp waisted bolt shroud on this Achiever.