It's RCBS Part # 09111
They call it ' Shell Holder Ram Clip Spring'

It's the same part on several different presses, some of them now discontinued but they still service them (if they were open of course)

I think from your description you have the Junior-3 press from the cast in ID.
It was also sold as the Reloader Special,,maybe a couple of other renditions.

The part # and name fits those presses as well as several others ....(Reloader Spcl 2, 3 & 5,,Partner Press, Rockchucker and Rockchucker II.
Maybe even more.

There's one of the parts on FleaBay now.
I don't know if some of the reloading places like Graffs and Midway would have parts.

Some people say to just use a rubber band to hold the shell holder in place on the RCBS press and it works just as well.
Might be an alternative to get it up and working while you search around for the part and not get into paying a crazy price for something a 2cent rubberband will accomplish.

Hope this helps..

added.. (While I was typing) I like RodD's idea of an O ring to hold the SH in place. Probably easier to manipulate and roll into and out of place on the ram than a rubberband.