Because SD-100 is post war vehicles. I might wrong because I don't know when did Czech build their SD-100. I know that IS-3, T-10M, Su-122-54 and mostly T-54 got that kind of back smoke grenade like BDsh-5 or (БДШ-5). The info about interesting smoke grenade is here Quora.com
Would that back smoke barrel can be use as smoke launcher?
Oh you mean the tanks at the very back? Those are extra fuel tanks. Most post WW2 russian tanks had those added.
Because SD-100 is post war vehicles. I might wrong because I don't know when did Czech build their SD-100. I know that IS-3, T-10M, Su-122-54 and mostly T-54 got that kind of back smoke grenade like BDsh-5 or (БДШ-5). The info about interesting smoke grenade is here Quora.com
Maybe it is a smoke grenade launcher, interesting.