There should be two numbers on the case back, one is the reference and the other is the serial, unless they have been polished away
I am not familiar enough with this time period to identify from the front, not sure if anyone else here can, maybe you will get lucky
It'll probably be 8421XX (I have no idea what numbers the "X" would be). 8 as it is steel and 42 as it'll most likely have a cal 42 in it. Most dial refs (the last 3 parts of the number) start at 100. There are quite a few in that case shape that were obviously part of the same collection. I've only ever seen that dial configuration once before and it is owned by a chap who goes by "timeis_universal" on IG. He may be able to give you the full ref number.