When you remove your current grips, what is the writting on the frame. Or if you go to the Ruger website you can search down what year it was made and the will help also.
Ruger has 3 common frame sizes you could have. If it doesn't have a lock under the grip your choices will be 2, If it's not the 50th anniversary model, then you narrow it down to 1, (Unless it was switched by somebody like me)
Although funny, the barrel size doesn't matter. Unless it's the Super Blackhawk with the 7 1/2 or 10 1/2 barrel