T-38 has 2 vent holes in the receiver ring/ T-99 has 1 hole
T-38 has ejector box milled in receiver/ T-99 E-box is screwed on
T-38 could have been converted to various calibers and would not be noticeable
T-99 most common conversion 30-06, but would have the receiver milled to fit longer cartridges--Many converted to .300 Savage, but the chamber area of the barrel is short from moving the barrel in.
T-38's are almost always non-matching. The MUM means nothing on T-38's the Chinese had tons of them and dumped them on the U.S. market over the years. I have had quite a few with really ugly barrels that shot really well. Check out what the receiver ring markings look like on the net. I had a lot that were modified for experiments and they stamped over the original markings. Stay away from those if you want a shooter.
Get online and look at some T-99 models. After 43' they started cutting corners. Try to buy something with a full stock and markings on the receiver above the MUM and stay away from the late war models if you want a shooter. Near the end they threw together "piecers". Usually if it has a chrome lined barrel it is early war production. It is common to find all matching numbers on a T-99, but if you are looking for a shooter it drives the price up and does not make them shoot any better.