I have one that came with my 1/4" drive S-K breaker bar. No P/N on it. It is 5/16" diameter and 4.5" long. It is older than 1960 but I am not sure how much.
I just checked the S - K catalog from 1965 on another thread and it did not show the leverage bar or the hole in the breaker bar for it.
This is one of the first pieces that gets lost out of a set because most people don't know what it is. Most manufacturers stopped providing them in the 70's. It seems stupid that a simple bar can be the rarest part of a set, but there it is. They can be really hard to find now.