Before looking at the auction, I thought it was going to be one of those foam "dolphin developers kit" toys they gave away at GDC.
I can't say where I've seen the cartridges before, but I'd say this is authentic. They used the cartridge format to make instantaneous "burns" of each new game build for quick testing, and it also simulated the Cube's quicker-than-normal load times (but with some artificially added time if they wanted a more realistic estimate.)