I know that the lenses for the Fives Squared and Fives 3.0 are compatible between the models. Are there any other frames that use the same lenses as another frame?

Splice and Unknown do, as with the Flak Jacket and Half Jacket 2.0. I believe the M2 lens can sort of fit certain M Frames, but it's not perfect. Original Half Jacket lenses can sort of fit into a Romeo 2, but again it's a matter of fiddling with them, and it's not exactt.

Thanks Dann. Out of all those, I do have the Splice and Unknown. I will have to try those out since I have black Iridium and grey for the Unknown and Ice, green and purple lenses for the Splice. Out of my collection of 20+ pairs, the Unknown and Splice are 2 of my favorite looking frames.
I also have 2 pairs of original Half Jackets but no Romeo.

It was useful back in my custom swapping days. Ruby Clear and the G30 lenses were fun to throw into the Splice frames.