You see it a lot in watches of that sort of age, the grey market was pretty rampant for a time. Nowadays the ADs are much more amenable to those sort of deals themselves to help make a sale, but back then (few short years ago, perhaps, but economically quite different!) they were standing firm at, or very close to, the RRP/MRSP.

A salutary tale, I guess, but the watch is still as good as it would have been, just minus that number - that only affects resale should you come to pass it on in the future. For a while, some of the manufacturers were refusing to service watches without serials, in an attempt to discourage grey market sales but most if not all seem to have retreated from that position now.

I'd be even warier of buying a caseback by itself, as there is a much shadier practice of cloning casebacks from watches with limited edition numbers. A lot of different fakes turn up with the same caseback, with the same serial/LE number. It would appear a run of casebacks is manufactured and then distributed amongst a number of fakers - seen it with Omegas a lot as well as the TH re-issues.