Ray,
Indeed rollover bars certainly began to appear and grow during 1961.
For example;
January 7th 1961. NZGP at Ardmore and Stirling Moss in his Lotus 18 # 7 has no form of rollover bar at all.
By May 1961 at Monaco the Lotus of Stirling Moss had developed this roll bar.
By September 1961 at Mosport in Canada Stirling's Lotus 19 had a roll bar as shown.
Also at Mosport in 1961 was this Lotus Seven with a roll bar that looks similar to the one on the Lotus Seven on sale in San Luis Obispo, California today.
(So there is my input on the development of roll bars during 1961. )
(Ken H )
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