I checked out your photos. The light purple deck looks nice and is certainly the cleanest side of the board. Neat shape with its wide nose and tear drop template, ending in the scouped out diamond tail. Dual channels on the bottom, inside the outside keels (runner fins) and the center ridge fin. The channels start deep and wide at the tail tip, converge in at the runner fins, and then widen out and gradually flatten as they approach the middle of the board. In theory, I can see the water rushing along the bottom, concentrating and accelerating as it flows through the narrowing channels (venturi effect) and out the back. I think that's how it was supposed to work, but I'm not sure the average mortal would notice it.
Mike Eaton tells me that he made quite a few Bonzer kneeboards in the 1970s and later. Your friend's Bonzer is an early one, either 1973 or 1974, based on the pointy tipped runner fins, which later were rounded off (less dangerous when you're swinging your legs around under the board).
Do you live in Texas by any chance? Bing remembers sending a few of these with rope handles to the Bay Surf Shop in Houston, Texas. I'm not sure how many were made, but I think they're very rare. I had never seen one before until about two months ago when a really clean one popped up and I bought it. Now another. Funny how things go.
It might be hard to see because of the opaque purple deck, but maybe you can find the board's serial number on the deck just up from the tail. And, look for the shaper's signature on the stringer next to the number. If there is a signature, it'll either be Bing's or Mike Eaton's signature.
This one is fair condition (I'd call it a 5.5 out of 10). I'll send you an email with what I believe it is worth.
Thanks for sharing this great board with us.