Greetings,
                  I picked up a Cavalry Arms C1 stock, which is a new-production stock along the lines of the old M16A1 stock.

The buttplate and rear sling swivel on the C1 were a bit cheesy, so I bought replacements off the EE.  Rear sling swivel replaced just fine.

But the SP1 buttplate was not quite the right fit:  the toe on the buttplate doesn't extend down as far as the bottom of the stock, so there's some edge there.  Not a ton, but enough so it's obviously not a proper fit, though it should function okay.

Any tips from those who've replaced CavArms buttplates?  Do I need a proper M16A1 trapdoor plate, or does the A2 trapdoor buttplate fit even better?

    -MV

Not sure about A1 trap doors but I do know many people put A2 trap doors on CAV C1/A1 stocks.  I don't see why a real A1 wouldn't fit either.  Most people probably go with the A2 style because they are much easier to find.

The best fitting buttplate for the Cav stock that I have found, is the buttplate from a Bushmaster Stubby stock.
I was disgusted with the POS buttplate, and rear swivel as well. The Stubby plate doesn't have the trapdoor in it.
An actual A1, and A2 plate will fit the same as an SP1 plate. Not quite long enough to extend all of the way to the bottom.
The Stubby plate actually is longer than the stock by just a a slight amount. Maybe .020 or so. Not enough to be noticeable.
I don't know for sure who makes the Stubby stock for Bushmaster. RRA maybe? If they do, you may be able to get a buttplate from them. I had the stock from a past build I did, that wasn't exactly what I wanted.

Any tips from those who've replaced CavArms buttplates?

Just sand down the stock to meet the contour of the buttplate. It takes all of 10 minutes.