Johan –
Not sure from your photo quite where on the lathe that number's stamped. On mine it's on the end of the machined vertical surface of the shear, just below where the tailstock is usually parked, and I think that was Myford's normal practice and mentioned in the operating-manual.
I don't know if the longer numbers normally had hyphens. The Lathes.co list does not quote hyphens but the compiler might have been left them out.
Still, it does look like a serial number, but as I say the number you quote, with 6 digits, would suggest your lathe was made in the early-1970s; more recently than you thought.
It's not been re-ground, well, not by Myford. They designated that by adding an 'R' to the number; but that's not there and anyway, from what you tell us of the machine's state the previous owner wouldn't know a re-ground from coffee-grounds.
'
Missing teeth from the headstock gears is often from someone habitually releasing a tightly screwed-on chuck by jamming the spindle with the back-gear, and hammering a makeshift lever (such as the chuck-key) in the chuck. Once he'd broken a few teeth the remaining ones came under increasing unfair stresses, especially if he worked the lathe unduly hard with heavy interrupted cuts – whose shocks tightened the chuck even more onto the spindle…
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As for abused machine-tools….
A Bridgeport turret-mill and a big Dean, Smith & Grace lathe obviously used for machining…. sandstone!
I spotted them while on a geology-club tour of a masonry-stone quarry and works. There are special milling-machines and lathes for masonry, but these forlorn specimens were not them! They'd probably come second-hand and cheap from an engineering works.