Identify this old fin box

Hey Friends. I’ve been gifted an old (thinking early 80’s maybe late 70’s) single fin. Issue is that the fin box is unlike any I’ve seen. It’s about 9 1/4" in length and has teeth or ridges inside that once held the fin. Apparently, the fin had ridges or teeth that would lock and hold the fin in place. But that fin is long gone. I’ll likely replace the box but I’m curious if anyone knows if there are compatible fins for this out in the world still.

Heres a photo:

Guidance Fin System. Occasionally one pops up on eBay for sale.

I have an old Hansen with what looks like the same fin box, there is some discussion on the topic in the thread here 8’ 2" transition era Hansen with “Ventral” fin & box - General Discussion - SWAYLOCKS

I think its a “Ventral” fin box - bottom line is that there are very few fins out there, and if you find one, it will probably be a crap shape, and very expensive. I think Bird’s surf shed has a couple, but way too pricey for what you get IMO. You can make your own, as I did in the pic I posted, or as you say, just remove the box and put a standard fin box in, which seems the most practical solution.

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