Who knows...

[gash mcslash]

Any guesses?

http://providence.craigslist.org/spo/696575498.html

[sfw]

In the early 60's, Diwain Surfboards, Huntington Park, CA used the same fins on their "ding proof" surfboards. Diwains were made of two fiberglass shells, sealed with a rubber bumper all the way around the rails. I don't see the bumper in the photo, but it's a place to start.