Should be a C7.....but could be a frankensaw built from various C-Series parts. The C7/71/72 and the C9/91 were the ones with the red chassis. C9 and C91 usualy have green starter covers, AF covers, and clutch covers. Again, somebody could have mix-n-matched things. "Convertable Drive" was a decal on all of the C-Series saws. "Convertible" is where the "C" in the C-Series model designations comes from. Guys lately have been calling their C-Series saws "C-51 Convertible" (for example), when "convertible" is redundant in the name.
The "Convertible" thing was a marketing strategy. These saws could become a gear drive saw by having a special planetary gearbox installed in place of the clutch cover. There was also an auto-oiler assembly that could be installed in place of the clutch cover on the C-Series and early XP-Series saws. Those attachments are rare, coveted, and somewhat expensive when found for sale now...
BTW-Measuring the bore will get you 1/2 of the way towards an identification. C5/51/52 and C7/71/72 saws have the same bore. The C7/71/72 have a longer stroke. The C9/91 saws have the same stroke as the C7/71/72 saws, with a larger bore. Most (but not all) C9/91 saws have a different intake manifold than the other C-Series saws. This manifold has a 'pyramid' style 4-reed setup, where the regular C-Series saws have one flat reed. The last of the C91's (after a certain serial # that I can provide if anybody cares) have the single 'flat' reed intake manifold as used on the other saws. My guess is that this was a way to 'widen the gap' between the C91 and the newly introduced XP1000 saws.