Okay, I took them apart, and it seems like everything but the housing is interchangable.

The housings are marked right and left.

The piece that holds the reverse lenses in are not marked, but they have different part numbers.
The back up lenses are identical.

The brake lenses fit in both housings. but are totally different part numbers. Not just odd and even for the housings and backup lens housings.
But they are both odd numbered part numbers. One ends in 911, the other ends in 907.
One fits better in both housings than the other.

Was this just a different run of parts? Or did someone stuff both left lenses in my car? It isn't marked left or right like the other stuff.