Luke, I think the change may have been a little earlier, more like circa 1966. (Working from memory here.) Colt shortened the grip-frames of the D-frame revolvers across the board as a manufacturing economy.
A search at www.coltforums.com
may turn up useful info for you.
Any source selling "late model" or "short-frame" D-frame grips should have what you're looking for. The only other short-frame D-frame stocks would be those specific to the Agent model as produced ca. 1955-66, and would generally be specifically marked as such. (If you run across a set of THOSE, let me know, 'cause I could use a pair for an old Agent I have.)
Hope this helps,