SAB, you had already come to the right conclusion, that they both use Picatiny rails, therefore what fits on a RAS/RIS will work on any Picantiny mouting device, as long as it has enough grooves
I have a RAS, used to have a RIS too. On the RIS the index numbers start from the front sight, on a RAS they start from the rear, 14, since some flat tops have these numbers that go from 2 to 12, so the RAS picks it right up at 14. You're correct on the quick remounting, if you ever need to remove a mounted device, it's easier with a numbering system so you'd remember where it used to go. I had that problem all the time, I never remembered which grooves my scope used to use
The index numbers are on top, bottom, left and right, for instance, top is T14, T16, etc., bottom is B14, B16, right is R14, R16..and so on.
I don't have a Quad-Rail, my rifles all have front sights already attached, it'd be a pain to remove it to install either a Quad-Rail or a free floating RAS.