Just to add to the confusion. Tyler says the 'N' frame takes the #4 t-grip. For square OR round butt guns.
I have a model 21-4 (Round butt 'N' frame) and the #3R ( round butt 'K' frame) T-grip fits like a glove. Much better than the #4 T-grip.
T 1 Tyler for J frames fits J, K, and L frames round or square butt. In some cases The T 1 will fit a Colt D frame. The J Tyler is a bit narrower than the K, the arch is the same. The arch is the big determining factor. I usually only buy T 1's. Old farts like me have always used the things. It's almost unbelievable what auctions are getting for them. The short butt Colt below has a T 1 on it. If you put long stocks on it you need a T 5.