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.