This issue of the rifle serial number on the butt tenon on Savage rifles has come up recently in another thread. My rifles have been so marked (last four digits) & another forummer who owns two Savage T's removed the butts on his to check & found the same thing. A few other people who have these rifles have noted the same too. I suspect this practice (at H&H) must have started before the practice of marking the T, S etc. However, early Savage conversions do seem to the sport the S51 on the butt semi pistol grip, which I have taken to indicate that this also appeared earlier than the other 'typical' markings. Obvious exceptions to this (bearing the S51) would be any RSAF Enfield converted Savage rifles. I would agree absolutely with lee enfield that if I saw an early Savage with T, TR, S & so forth, I would be looking at it VERY critically.
2 of my Savage No4Ts appear to retain the original butt stocks - I guess I will have to pull them to see what I find.