Mike Hailwood's first ride on a Ducati 750 - or is it a 500?

This photo is one of the many excellent images avialble from my favourite historian Ivar de Gier, who trades on eBay with his wife Amy as A.Herl Inc. Ivar's always been 100% right in my experience, but this time it's difficult to be certain. The photo was taken at Silverstone in August 1971, and was a test which explains why Mike's in the leathers he wore at Daytona racing a BSA/Rob North Rocket 3. Phil Read was also present testing the Ducati 500GP, which is what Ivar thinks this bike is. But is Mike really on the 500, or an early 750 as usually claimed in photos of this session? Of course he could have tried both, especially given that the 750 being tested was an early racer based on the 500 rather than the 750 GT which was rather sturdier as it was intended for production and road use. All thoughts gratefully considered. And I'll email Ivar