Is there such a thing? Looking at foilboards and the one I really want doens't come with any fin mounts but does have both tuttle and track type foil mounts. Would like to be able to mount a fin and use without foil for SUP and windsurfing.
No need for an adapter, as the Tuttle box is originally a fin box. Just find tuttle fins, there should be many second hand windsurfing ones.
Note that there are 2 standards, tuttle and deep tuttle, and deep tuttle fins are generally huge, but there are exceptions such as:
I think most foil boxes are deep tuttle. For instance the Go foils are deep tuttle... but not all, check.
But you can put a tuttle fin in a deep tuttle box:
Also, you can find cheap plugs that are used to close a deep tuttle box when not in use. i guess you could route a slot in it and glue an US Box fin in it.
Check out the general windsurf forum as there is a recent discussion titled "fin adaptors".
It may require machining a us base fin down to fit.
In the early mid ninties some built our windsurf wave boards with Tuttle boxes so there were some fins but most windsurf wave boards went back to the us fin box for adjustment for aft.