Fin for Aitken 7'6" board

[mpm32]

Hi, first post, super newbie here. I always wanted to surf, tried it a couple of times. I do other on water sports, water skiing, barefooting, wakeboarding etc.

I finally got a house on Cape Cod and there was an Aitken board left behind in the house. It's in good shape but it's missing it's center fin of the three.

The fin system is the kind where the fin has a short roll pin in it that slots into the finbox and then you secure it with a screw in front of the fin to a square tapped metal piece that's in the finbox.

I obviously don't know what this fin system is called so searching for a replacement fin has only turned up longboard fins that are not shaped like the rail fins although they have the same type of mounts.

Can anyone point me in the direction of a replacement fin?

Is this board at 7'6" considered a longboard? I know it's probably a beginner's board since its roto molded poly. Which would be fine for me at this point, I think.