Very nice 1/25 T-20 - interesting vehicle which hasn't been modeled in paper as far as I'm aware.
I'll presume to fill in some details about the T-20. Designed in the mid-30s as a light artillery tractor - apparently quite successful in that role - the seats were for the gun crew. About a 100 or so were converted to SPGs by removing the seats and adding a 57mm or 76mm gun - these were used extensively in the battles in front of Moscow in 1941.
All it needs is a 45mm or 76mm model gun to tow...
Are you going to release the model commercially? - it certainly looks good enough.
Regards,
Charlie