I acquired my second Parker 51 this morning, perusing a local vintage fair. It was manufactured in England according to the cap, and it's in reasonably good nick. It's a Mark III and judging by the jewel, it was probably manufactured by 1973 at the latest, unless the factory in England did not bother updating the design to comply with some US regulation about removable small parts. It has a lustraloy cap (not sure about the spelling) and the barrel is black. There is some evidence that white letters were printed on it at some stage but they have mostly worn away. For this reason I think it might have been a corporate gift pen.
The pen still had some blue ink in it when I got it home although it flushed out easily enough. I've filled it with Edelstein Mandarin and will flush it out again when that's finished as I suspect there are still some dry flakes slowly dissolving. It's cleaner than my first Parker 51 however which is much older and looks like it has been constantly filled with black ink for most of its life.
The Mark III looks to be a medium nib and it's very obviously broader than the older pen. It cost me fifteen euro so all in all, unless I discover in the next week or so that it has a horror of a leak problem, I think I probably did pretty well for it.