That is a good observation. I also have a couple of Cyma/Tavannes watches with 5-digit numbers on their case-backs (see below).
I believe that Longines' case production in the era of order numbers was a mixed bag. Gold cases were produced by various manufacturers, which can be identified by maker's marks. At least some waterproof references were almost certainly outsourced, as they closely resemble cases used by numerous other brands. I could be wrong, but I am under the impression that Longines also manufactured some of their own cases. With regard to 5-digit numbers on case-backs, I am doubtful that there is a connection between Longines' order numbers, and Cyma/Tavannes' numbers. But who knows!
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