To be honest I haven't researched the early versions as much as I should have; I was content to wrestle with the ancient Skif T-64Bs really just because I had them in my stash, and I am never minded to replace like for like just because a better kit comes out (clearly I have a perverse nature!). I have the Skif T-64A also and will attempt to do that justice although am more than aware that for the expenditure incurred in adding components I could have purchased the Trumpeter version in the first place!
Anyway, as for the 115mm armed version: I only have 2 x reference pictures and they are from the Osprey book on the vehicle. The text and photos by the way are excellent, the illustrations less so, however, that's besides the point. I note from the 2 x photos of the ostensibly 115mm armed version (2 pictures of the same tank) that it appears as though it was equipped with a stereoscopic rangefinder as there are 2 x identical housings - presumably for the lenses etc - at an equidistant spacing on the turret. I also note that this arrangement is also evident in the early part of this film clip (which also shows the Schnorkel arrangement on the hull rear):https://www.youtube.com/watch?v=cfKnVd51ODA
Dig in at around the 2 minute point as the footage prior to that is of an even earlier version I think. I also note that the tow rope arrangement on the glacis is different in that both tow ropes are present.
Sooooo, from a modelling perspective if I were undertaking this (which I now might as you've whetted my appetite!) I'd probably make the following mods (this is not exhaustive just really shooting from the hip a bit):
a. Remove all details from the glacis and reconfigure as necessary ie adding towropes and the apparent single "bar" in front of the Driver's position. I'd recommend the Karaya tow ropes if you can get them.
b. Pare the turret right down, sanding where necessary ie no MG, removal of any lumps/bumps that do not show up on the pictures/videos. Add the apertures for the presumed stereoscopic rangefinder from spare etch/plastic card.
c. Add tarp to the turret rear - always a Soviet necessity.
d. Obviously add 115mm gun to taste (how the hell did this work as the turret was designed to deal with an automatic loader not big fixed-round ammo?)
e. Fix Schnorkel to rear hull.
You might wish to invest in an etch set for the finer details not least the grills/louvres at the rear and a few other details. I think Eduard still produce something which covers these - I'm afraid I haven't checked.
I'm also painfully aware that I may be guilty of teaching you to suck eggs - so apologies for probably far too much supposition - but I do love the T-64!
'Hope this helps somewhere along the line.