You can't interchange these anymore, the frame it cut differently due to the new Recoil spring, the trigger bar is different too..
On a side note, the RTF2's are changing their serrations to the normal straight style.
The 37's are coming out shortly and the 19/23/38 Gen4's will be introduced around summer of this year. And, I haven't heard plans to Gen4 the Sub's, which obviously there is no reason for interchangeable backstraps for subs.