Zac,
Definitely should be TF; either a miss-stamp, extra chisel mark, or who knows what. The letter font used has serifs, so could you be seeing the 'foot' of the 'F'? From about 1921 on, frames were prefixed AF and worked their way through the alphabet to ZF on a special model supplied to Poland in sometime about 1930. So all frame prefixes ended in F. Thereafter the frame prefixes began with F, with FA being a 1930 350cc, and again right through the alphabet (skipping a few letters this time, or a few went undocumented) to FZ on a 1935 500cc sold to the War Department. They had quite a variety of models in the early thirties! Having exhausted the two 'F' combinations, they then used AA, AB, AC, etc through AF to see out the remainder of the thirties. There really were not any new production frame designs after 1936. The only thing recorded outside this range was a F.S.S. of 1935 used for a single customer.
The V5 is an inspector's stamp, it is pretty common.
Generally I understand straight 50 weight was the equivalent to what was used in the day. However since it is a total loss system, practically anything will do as it will only pass through the engine once! Fulton used mustard oil in a pinch on his round the world epic! So the usual condemnations about using multi-grades in roller bearing engines does not matter as much. The oil does not stay in long enough for the chopping up of the polymers to be much of a factor. It really comes down to a matter of the condition of the bores and rings, and how fast the oil gets past into the combustion chamber.
-Doug