This M7 Ambassador has a 3-digit number  stamped inside the case back and a different, longer number (1-542325), stamped on the outside. This appears to be the case on all examples of this model. I have seen numbers such as 500 and 524 on the inside of other examples. Those other examples (so far as I could examine them via online photos) also had different numbers outside the case back.
I do not know the reference number of this or how it was described in catalogues of the time to differentiate it from other Ambassador 30j models.
Movement is 12EBACD, also marked SX and D
Inside caseback is stamped CB within a tombstone-shaped cartouche. I do not know whether this indicates the case maker.