Although the part of TFS is 51504 for their amber parking light housings, both the LH and RH housings show the same 51500 physically on them. Oddly, the housings for the BTR clear parking light housings also show 51500.
There isn't really much to difference, other than the lens colors. The backs of the housings are identical chrome:

They are the same width:

The same height:

The lenses are both flat and the same thickness:

These are both LH housings, in case you're wondering, but you can vaguely see the markings in the first image.
Since I have other lenses, I gave these a fairly good test to see if the lenses came loose. My original TFS clear lenses seemed fragile, right out of the box. Both the TFS amber lenses and the BTR clear lenses seem to be quality products.
I'll have to dig the other lenses out of storage, one day.