No way of knowing how many were made. Anything is a guess. Ruger does not/will not release production figures.
I have several very nice letters from Ruger over the years that simply state. "Production figures are consider propriatary information and we do not release this information." Or something very close to that.
An RL has a thinned down stock with a black/ebony cap on the forearm that is absent on the other models of 77 in 257 Roberts.
The 257 Roberts is certainly not common but I would not consider it rare.
That being said they are nice rifles and desirable.