Five of those killed in the MM accident were children so maybe that had something to do with it.
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As best I can find, the first originally Indy 500 driver to compete in a regular Formula One World Championship Grand Prix race was Troy Ruttman who finished 10th in the 1958 French Grand Prix driving a Maserati.
That appears to be correct. Presumably the ride came about from contacts made at the Monza Race of Two worlds in 1957, where he won one of the heats. Surprising really is that others were not signed up. Roger Ward was interested and qualified, but his entry in the USGP the following year was self initiated with a midget car. This was to be Ruttman's only F1 start, he entered the 58 German GP but DNS. Travel times and low contracts probably discouraged many Indy drivers from perusing more F1 races. The talent was there all along.
I believe this 1959 U. S. Grand Prix was the first and last time that an Offenhauser engine ran in a regular Formula 1 World Championship Grand Prix event. The Goossen engine in the 1960 Scarab was a 4 cylinder engine out of the Meyer-Drake-Offenhauser organization but I don't think it could be called an Offy because it had a desmodromic valve system. I also believe that it was not a fixed-head or 4-valve engine - other "musts" for an Offy.
Here's a "Ward-type" Formula 1 Offy engine-great sound! (full volume and headphones recommended)
https://www.youtube.com/watch?v=-FM5JwuOgPI
And on the topic of lasts, I believe the 1960 Scarab was the last front engine rear drive car designed for Formula 1. Although out of date by the time it arrived on the grid, it was a neat little car-basically a 7/8 scale layover Indy roadster.
The last front engine car in F1 was the 1961 AWD Ferguson P99; another 4 cyl. (Coventry Climax this time) little honey.
Firing up the Scarab-America's first Formula 1 car:
https://www.youtube.com/watch?v=aur0tH--6BM
Roger Ward's Kurtis-Offenhauser that he ran in the 1959 U. S. Grand Prix World Championship race was built in 1946 so I think it was probably the last car to run in Formula 1 that was actually built before there was a Formula 1.
In a manner of speaking, the first and last time a midget racing car won a Formula 1 World Championship race was Lee Wallard's "stretched midget" in which he won the 1951 Indianapolis 500.