This is an interesting one. I don't know a lot about this, but a quick bit of searching says that the Roberts frames of this era where built by Jerry Griffith in Oakdale, CA. The silver frames were ordered by Penton, for, presumably Sachs 250 motors (which is backed up by the eBay description, though the seller says early KTM. That's partly right because KTM bought in Sachs motors, and the majority of Pentons were basically rebadged KTMs for the US market). I read that only 41 of these silver frames were made, around 1976-77.
The seller says 'it could be modded to fit a 450' and that is right, but it would be a shame to weld the frame to fit a radiator that a 450 would be need. Plenty of air-cooled, two-strokes would fit. It's a neat piece of flat track history. But it now for £1150 or make an offer. Here's the link and the description (that I haven't corrected). Thanks again to Kev for the tip-off.
Kenny Robert Flat Track Frame
A very rare Kenny Roberts Flat track frame. This still new although stored for long time,I'm now 90% sure that frame is built for a early KTM 250 engine although got RD400 on the Tank, or could be modded to fit 450 engine . It comes with brand new fibre Glass Tank, with cap and p Taps. It got rear axel and adjusters, 4 shocker mounts on swinging arm, Got Bates foot rest holders ,new rear Brake pedal, engine Bolts and steering head races. Great winter project and very rear chance to buy Kenny Roberts frame .