You are certainly a lucky guy. I have three no dash M48's, but only one has the 3 T's. Tried to letter it but it came back as a 6" Pre M14. Mr. Jinks rechecked and it appears to be a S&W recording error.
My 6" is # 368119, 106 numbers from yours, and has no Xtra features, but it is near mint, in its original numbered box. It was shipped 7-16-1959 according to the factory letter. 2nd month of production.
It is well worth a letter to find out about your revolver.
BTW, the box is an over stamped K22 box. I guess they ran out of MRF boxes. It is numbered to the revolver. The boxes are very hard to find from that era. Big Larry