...but obviously not enough to actually replace the upright...
Are you being willfully thick here or what?!

You're falling into the same trap as Munroast and the others who seem to assume that once a car's first owner sells it on, it somehow magically disappears! What actually happens, of course, is that it passes to another owner (that's how you got your old shed, remember)?
What it proves, is that you really don't understand the ISO screw thread classification system do you?! I thought the hint I gave you in my last reply might have caused you to stop and think about it - maybe even have a quick "Google" in an attempt to educate yourself, but it seems I misjudged you. Apologies. Once again, I expected too much...
Not that I imagine any of this will stick, but there's an international standard for metric threaded fasteners. Anything that doesn't conform to that standard is er... "non-standard" (the clue's in the name...). It doesn't matter how commonly they are used in any particular application, that has absolutely nothing to do with it. My own car has a number of M7 bolts and cap heads in its engine. They made lots of those engines, but that doesn't mean they're a standard bolt! (Oh... hang on, how could I possibly know that?! I've never even unbolted a wheel)!