Question about Soviet Spam Cans

Hi Everybody. First post and already a picture.

This maybe be a stupid question but I am at a loss and someone from TheHighRoadsent me here.

I have a 7.62x54R spam can and am getting info on it.

I know what everything on the can means except for the powder type and source. I have powder type BT (VT) and source C (S).

Cant find any information about these two. rest I know is 7.62x54R lightweight steel core, copper washed bimetal case mfr in Novosibirsk in 1977. powder mfr in 1977

Any info?

Ive checked and cant find anything on the powder type and source

Thank you