Like Jim mentioned the 7 1/2's are Small Rifle Bench Rest Primers which are pretty much the std one with a stronger cup and supposedly more consistent. I use them quite a bit and have noticed no real differences in end use performance or pressure signs. However the stronger cup may make reading early/less drastic pressure signs a bit more difficult.

That said, just back your starting load off a bit more than usual and keep a close eye on things approaching max loads a bit more carefully.