When I'm awake enough I defer questions like this to Modem7's website as cited above in post #7. But it's worth while to note what he says near the top of the page:
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"Some people refer to the early versions of the 5150 as the 'model A', presumably because later versions have a large 'B within a circle' stamped on the rear (example).
I don't recall IBM ever using the terms 'model A' and 'model B'. They used model numbers, e.g. 003, 104, 164, ..."
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It may well be that IBM used up existing stocks of parts then switched to newer versions as they ran out, like VW did with the early Beetles, rather than wholesale upgrades at a clean model changeover point like other car companies do.
Just look at the power supply variations pictured at the link. If one was going to try and "restore" one of these beasts to a "period correct" configuration one would have to have some idea at what serial number or production date a given part was upgraded. AFAIK that kind of information has not been released by IBM. Add to that the fact that many of these machines have been tinkered with repeatedly for 30 years and you find people are "kinda sorta" reduced to making educated guesses based on chip production codes with large gaps in available information gleaned from the remaining machines that the owners are willing to inspect and publish what they find.
But even I can look at the back of the case and tell the difference between two screws and five. So! 2 screws -> early and 5 screws -> late. That's my story and I'm sticking to it! ;-)