Interesting nothing ... that empty half sprue! The T-26 was used as an experimental platform for many different variants. But most never saw production. And the flamethrowers (OT-26 which did have the turret on the left, and the OT-130/133/134) account for the majority of those variants (1,224).
So we are left with a very limited number (33) of SU-5s (self-propelled guns). Not sure half a sprue is enough to cover the gun and shield. Or how about the ST-26, which was a bridge-laying engineering tank. They produced 71 of those. I would bet that half a sprue is enough room for the actual bridge itself (which was 6 meters long) and the necessary support beams. There was also the Radio Controlled T-26, which might only need a few different antenna. The T-26-T was an artillery tractor. It would need a different superstructure, but the rest of the vehicle looked like a T-26. Half a sprue would easily cover the new superstructure.
Maybe it's not for vehicle options at all. Tank riders?