The CM, if you don’t know, is a rare variant from the mid-70s onward, by which time the FWB300, Anschutz 250 and the Giss Dianas had replaced the recoiling rifles at the top end of the market. So they were obsolete when new, or at least obsolescent. Meaning they are rare.
They are also nice, and their 1970s obsolescence is irrelevant given that none of the above has been truly competitive at 10M since the late 1980s. All are now for collectors, precision plinkers, or bell target.
Until recently, I’d have said £200-250. I still think I should say that, as that’s the price level that a re-sealed Diana 75 or an FWB 300 commands. Why should a slightly inferior Weihrauch cost more?
I have though recently seen nice HW55s going for quite a bit more. Maybe that’s coincidence on a few sales, or a fad, or one obsessive collector buying choice items. I don’t know. I’ve not seen the same thing with other classic target rifles, unless they are v special, like Walthers with tyrolean stocks and set triggers.
So my gut feeling is it would sell almost instantly at £200, and reasonably quickly at £250. Ask £350-400 and I have no idea. Maybe no takers, or maybe you strike lucky.
Sorry for not giving you an authoritative assessment, but I hope this explains why and gives you some food for thought.