Good evening!
(1) Was the AC 4880 fuel pump original equipment on slants with rebuildable pumps, or was it a replacement/aftermarket part?
(2) If it was an aftermarket part, does anyone know how it compares with the stock pump in terms of quality/durability/reliability?

No. AC is a division of General Motors. AC pumps were not original equipment on any Slant-6 engines. The '60-'63 engines came with the Carter M2996S rebuildable pump.
AC's pumps tended to be pretty good. Not possible to answer more specifically without seeing exactly what pump you're asking about; that "4880" number was applied to a lot of different pumps over the years.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thank you, Dan. I was just following an eBay clickhole into rebuildable fuel pumps for the slant six.
https://www.ebay.com/itm/1960-61-62-Ply ... 1853427622
https://www.ebay.com/itm/1960-61-62-Ply ... 1514153082
https://www.ebay.com/itm/1960-61-62-Val ... 2326879127

There's a gentleman selling a Mopar AA-4880 / 2525679 pump on FABO:
https://www.forabodiesonly.com/mopar/th ... mp.447911/

I'm guessing any of the above would do the job; just wondering if any of them is especially desirable or not-so-much.

The carters come up on ebay fairly often. I bought a NOS one for $85 shipped last year. Lots of them are overpriced you just need to be patient, and I don't see the need to buy one that has been used as they are still common to find in NOS form.

NOS ones still need rebuilt.

http://www.then-now-auto.com/fuel-pumps/ is the recommended place to buy rebuild kits.

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https://www.flickr.com/photos/162600228@N03/

There are good reasons to assume OGLeanDart270 is correct that a NOS pump would want rebuilding before use. The obvious one is diaphragm and valve compatibility with current-day fuels. That said, I used enough NOS pumps with modern fuel, without problems, not to worry about it much -- I just operate the lever with a finger firmly over the inlet port, then with a finger firmly over the outlet port, to make sure the valves aren't stuck and the pump pulls vacuum and holds pressure, then toss it on. The less-obvious reason to rebuild is that some 1963 pumps were made with improperly-hardened lever arm pivot pins. Eventually the arm saws through the pin, then the engine stops; TSB is here. Odds are against finding one of those pumps any more, but it's not impossible. Rebuild kit comes with new pin.

If I were going to buy any of the pumps you linked, it would probably be one and/or the other of the AC-boxed items. Nice prices on those, too. That said, I prefer the design and construction details of the original Carter pump. How much they cost depends on luck of the draw and buyer's patience. They are easy and fun to rebuild -- when you order your kit from Then & Now Automotive, be sure and ask them to double-check that they're sending you a diaphragm with a 3 to 5 pound spring on it; heavier than that is too much unless you are building a serious monster-motor.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks for all the info, guys.No luck finding an M2996S so far, but how about this marine/industrial Carter #4757? (Would the rebuild kit be harder to find?)

https://www.ebay.com/itm/1957-1972-Chry ... 2648262846

Should be just fine—doesn't look like a marine pump to me, and I wouldn't be surprised if you find M-2996-S stamped on the flange when you get it. Kit won't be any harder to get no matter what's stamped there.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Snagged it!