If that's a push-in type plug that's about 3/4" diameter, it's most likely a hole for the auxiliary fan switch found on a 92 Model 940 with the Regina fuel system.
Your 240 doesn't need the switch, so a plug was fitted.
This is a one size fits many radiator.

The plug is available at Parts Geek
http://www.partsgeek.com/catalog/1992/volvo/940/cooling_system/fan_switch_plug.html

You will also need to replace the rubber bushing too, as it will probably fail again if you don't.

http://www.partsgeek.com/catalog/1992/volvo/940/cooling_system/fan_switch_bushing.html

Do not lube the plug or bushing with any grease....push it in dry with maybe a little water to help lube it.


I made a temporary plug out of a piece of 3/4" PVC Plastic for my son's 940.
I used a bench grinder to grind the OD down to the proper size, and left a 1/8" flange on the end.

Since this is for a 240 Series, you should be able to plug the hole with anything that's close while you wait for the replacement plug.
Leave the radiator cap loose so the system won't build any pressure.
That's what I would do.

hope this helps
steve

Edit to add:
If the web site won't load, simply go to PartsGeek.com and look for a radiator plug and bushing for a 92 Volvo 940 with Regina.