I was at a gun show several years ago and there were old traps and some sort of firearm that they used to set on holes for Predators. Has anyone ever seen the latter or have pictures? I collect old photos of trapping and such.

You may be talking about a "set gun". I have known people that rigged them up to kill gophers in their farmyards using an old single barrel shotgun and some trigger activator like a cord from a mousetrap mounted to trip when the gopher appears and snaps the mousetrap to fire the gun. I remember hearing older relatives talk about old timers setting them up to stop repeated burgleries but since there are a million legal reasons to not do it (like firemen & cops getting killed by them)they've been outlawed for many years to install one in your home or business. I don't know about "agricultural use" to control vermin.

you are looking for "gettr guns"

You talking about the ones that shoot cyanide into the face of what ever trips it?

The cyanide getters were the M-44.

https://www.aphis.usda.gov/publications/wildlife_damage/fs-m44-device.pdf

I've heard stories of set guns used in cabins for bears breaking in and vaguely remember stories of small projectile devices at predator sets but will have to look that up some.

Here you go I just had to put a little effort into it. They were banned a few years before I was born so it is just memories I have and then have to confirm with an internet search.

https://www.guns.com/news/2012/07/23/set-guns-aylor-fur-getter-sure-shot

More, the "Getsem" gun

https://www.bing.com/images/search?q=get...ImageBasicHover