I think these very light (similar to Assos pedals), NOS, Stronglight PR300 pedals I found some years ago were prototypes too:


They came in an original unmarked, plain white box with specific cutouts to support the pedals that just had a few cryptic numbers stamped on them and the pedals themselves have no branding anywhere on them.
The very few pics I ever found of any other PR300's out there had Stronglight branding molded on to the resin bodies. These do not have them. I also noticed that the cage mounting screws on my PR300s are aluminum, while others I have seen in pics out there had chrome steel screws. The seller did not seem to know what these pedals were all about. I did not either, till I did my research on them, after I bought them.

I suspect that Stronglight might have meant to include these pedals in their Delta model line in the second half of the 80's, but prematurely pulled the plug on them because clipless pedals were starting to really dominate the pedal market already.