I have never seen any RB to SB conversion stocks for sale that were genuine S&W. However, I am sure that if you wanted to, you could use something like acraglas (sp?) to fill the voids. I was going to modify the set of stocks in the picture using glass bedding compound and fabricate a piece out of aluminum or stainless to fill the rear of the grips.
But then I found the Eagle web site and decided to buy a set of theirs and inlet a S&W grip medallion in them instead. They are not much more money than what a set of originals sell for nowadays and they look like some nice stocks.
BTW this is what my 629-4 Classic DX looks like with factory targets. I really like the look and they are very comfortable.