Very interesting ,
For comparison the Combat Magnum (Pre 19) was introduced as a 4 screw frame in very late 1955, except for a few hand delivered VIP guns the earliest versions begin to ship after the X Mas holiday in 1956 with 6 groove tangs.... 10 groove CM tangs seem to appear sometime around June-July 1956.

If my theory is correct your late production 5 screw pre 29 would be the only K or N frame target sight S&W revolver variant having a 5 screw frame with the newer 10 groove tangs.