These were very hard for me to find and I had a lot of help from a very kind benefactor who I won't name (snapmom). Without her help I'd still be looking no doubt.
An F670A flex round head and an FR42 which is even harder to find.
Honestly I'm surprised these were not more popular they are quite smooth in operation. The FR42 is a 42 tooth and Snap-on made one with double the teeth - the FR84 but I think it would be easier to track down bigfoot than find one of those.
This one was made in 54 and for some reason I really like this year mark the best (and no, I was not born in 54).
Ten years later Snap-on made this one (this was on my ten most wanted list for a long time).
The FR42 come from my favorite era of Snap-on. I really like the styling of the handles and the slightly flattened beam and many other tools of the era.