Hi Geoff--here is the earlier one with the steel breech http://www.muzzle.de/N3/Druckluft/Ba...baikal_40.html
And the late--ish right handed 53 here http://www.muzzle.de/N3/Druckluft/Ba...kal_ij_53.html
The 53M is even later and Ambi gripped.
You may find an earlyish USSR made 53 with an alloy breech block intead of the steel one of the 40 or the plastic of the later Russian 53's.
I hope you find a cheap one to tinker with.