Considering the IS-3 model in-game also gets the BR-471D round, that is apparently from the 50’s, I suppose there is justification for remodeling the IS-3 to reflect the most common configuration from the time period of the round.
An earlier model without this round could possibly fill the 7.0 slot. I disagree that it could be premium as you stated in your post. We don’t need to repeat the mistake of the T-10A by locking a service variant behind a paywall, when there are already plenty of prototype variants of the IS-3 and T-10 to set aside for premiums.