I picked up the saw today and yes - It definitely is a Frankensaw (for the time being).
It is an old Contra saw - has the ID plate 1106 - engine number etc on the flywheel cover, it has the same matching number on the casing - on the guide bar side, it has the muffler which has the top cover clip arrangement on it ( for the original contra cover ), and the fuel tank also has the clip retainer for the cover clips as well. It has the original bar ( i believe ) - stamped 1106 etc with the original stihl logo which is a good thing.
What I can gather has happened is that the original cylinder ass'y has failed, and that an 070 cylinder has been fitted to the saw. The decompression valve fitting has been plugged up - hence the reason why it is missing in the photo's. The air intake for the carby has been retrofitted to the 070 type as well.
All in all a bit all over the place - it needs an original Contra type cylinder, top cover, and air intake to get it looking back to what it should.
I am assuming that spare parts for these Contra saws are pretty hard to come by (?), so this one may end up as a parts saw. In its current guise, the saw has plenty of compression and seems to run pretty well, so will leave it as is until I either find a nice example of another large Stihl, or maybe the parts required to rebuild this one.
Regards,
Chris.