Its an old asymmetrical windsurf board, they were shaped that way to ride a particular wave with prevalent wind conditions. By shortening one rail line it made for more maneuverability on the riding (frontside) tack and the longer rail line made for better speed and upwind capabilities on the outbound tack. Check that your rocker will work as windsurfers have different rockers than surfboards. You chopped the tail off so you should be ok, there was probably lower rocker from the mastbase down to the back footstraps, behind the footstraps may have had a pronounced v and subsequent tail kick. The nose (forward from the mastbase) will have a lot of rocker (known as scoop to windheads). Is it PU or EPS? If it is PU it may well be an old clark windsurfer blank!
Im pretty sure its PU, it looksand feels and shapes just like PU foam. It actually feels pretty hard, considering i’ve planed about 2’’ off it!!. Yeah, the rocker is very flat, but hey, im gunna try it.
Here’s the fin that will go on it. I measured it, its actually 5x7, not 4x6. 
