So it’s normal that the bolts are all marked different. The tolerance that you measure with the straight edge was slightly large so I lightly lapped it in on a glass plate and „primed“ the pump with a little grease; I‘ll check for binding when I assemble…
It is 4.2 and it is different to your five.
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Except for the metric pump on the far left, I cannot vouch for the originallity of the cover bolts. I’ve had all of these apart for cleaning. I may not have put the same bolts back in.
I would think that your pump is an original 4.2.
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The two 3.4/3.8 pumps have 1/2" shafts. The two unmarked 4.2L pumps have 5/8" shafts on both the drive and cover side, the COUNTRY pump has a 5/8" shaft on the drive side but 1/2" on the cover side.
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Surprised Melling doesn’t make a replacement.
County (not Country) make lots of generic XK parts like pistons etc… Not sexy stuff or race-ready, but just regular engine reconditioning stuff. In a niche market, I guess newcomers would struggle to carve out a viable market share?
I can see that Bill Terry’s or Erica’s solutions can be quite adequate, but as a guy on only his 5th engine, I’m more comfortable with the serviceable used pump. I wouldn’t hesitate to use the new one if that was not the case. I do think the answer to the problem should be for manufacturer to understand that dowel bolts were used for a reason, and machine rather than cast the holes.
The practical matter is that that ain’t happening, so we have to figure ways to compensate. That’s why the interaction on this forum is so valuable - thanks, everyone.
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I’m a victim of Globalization! 
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My 160,000, OE, 1963 oil pump worked plenty good enough.
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