OK - I bought a -68 Speedmaster with what looks like a too-early serial number, as if it was from a 105.012 or even 105.003 (it's 24537xxx).
I applied for an extract and Omega quickly bounced it saying they didn't have the records; that and the fact that a respected watchmaker (who got the watch directly from the auctioneer) says it looks like it'd never been serviced or even opened, makes me convinced it's not a franken, but likely to be an errant batch of watches that were maybe using up old stock 321 movements with early serials, before the runout of the 145.012 and into the 145.022 with the 861 movement.
Is that credible? I'm sure I've seen 2 or 3 other watches being discussed that are marked -68SP on the case back, look like they stack up in other regards, but appear to have a serial number that is well outside of the expected range for a 68, at least if you look at MWO / ILMS or other well loved sources of knowledge.
If anyone else has a similar (24m - maybe even 2453xxxx as I know of 2 in that range) numbered -68s, please let us know for the benefit of the Speedy community...