appears some Pintle rings they sell are advertised as 3"... but they aren't (but are)..... technically. They are 3" somewhere..... but NOT at the eye. The 3" one I just bought at TLC is not equal to the 3" that fits the truck. Looks like the eye is actually 2.5"
You are right about that not all of the military Ø3"-tow rings (aka lunette) actually measure 3.0" inside. Seems to be a nominal size, only...
To confirm this, I just measured a couple of M116A3 - which have seen little to no use: inner Ø = 2.935"; outer Ø = 6.250"; ring thickness = 1.658"
We are talking here about the eye being only little over 1mm too small, which would be very easy to grind to correct size, if... your pintle hook didn't already fit.
Now, if you got one with a Ø2.5" eye then it isn't a Ø3.0" technically, that's clear or? And it would be a lot of work to enlarge to 3". I'd say, just return it and get the correct one.
This thread is somewhat related...
And here is a bit of
good info about what I think are US-made tow ring models and their dimensions... (pages 27-32).