There has to be some sort of logic to UN serial numbers. They surely cannot have made half a million wristwatches by the mid 1950s! I suspect that the first three numbers have another meaning. Could it be that "1xx" was for chronometers and deck watches with in house ebauches, "2xx" were wrist watches made and cased in Le Locle and "5xx" were wristwatches exported uncased? Here we have 234053, and jolly nice it is, but I have 5008928 which is only a few years younger and in an American case. Incidentally I think the little gilt anchor, absent from this watch, was applied after the signature was printed on the dial.