Don - would the initials by any chance be PR? I believe he won the 125, 250 and 500 championships and Mike won the 250, 350 and 500.

After reading Cargo's posting I was inspired to look up some race results in Mick Woollett's excellent book, Mike Hailwood, A Motorcycle Racing Legend. He lists these results for multiple race wins ( over 2) in one day:

1958

Snetterton .............3 wins
Castle Combe............4 wins
Mallory Park............3 wins
Snetterton..............4 wins
Crystal Palace..........4 wins
Cape Town...............3 wins
Pietermaritzburg........3 wins

1959

Snettterton............3 wins
Thruxton...............4 wins
Oulton Park............3 wins
Aberdare...............4 wins
Snetterton.............4 wins
Biggin Hill............5 wins

1960

Castle Combe...........3 wins
Aberdare...............3 wins
Aintree................3 wins
Brands Hatch...........3 wins
Silverstone............3 wins
Brands Hatch...........4 wins
Castle Combe...........3 wins
Snetterton.............3 wins
Aberdare Park..........3 wins
Brands Hatch...........3 wins
Brands Hatch...........4 wins

1961

Brands Hatch...........3 wins
Thruxton...............3 wins
Aintree................4 wins

1966

Czech GP...............3 wins

1967

Brands Hatch...........3 wins
Dutch TT...............3 wins
Brands Hatch...........4 wins
Cadwell Park...........3 wins
Mallory................3 wins

1968

Brands Hatch............3 wins
Snetterton..............3 wins

Sorry I couldn't make the columns come out straight I tried - but, it's harder than it looks LOL!