OK, those are used on every propane tank made from 1940 until about 1980 and are referred to as a "Senior" gauge and the part you are looking for is called a "dial". Go to your friendly propane supplier and they should have one like it. The markings will probably be different but the needle will still work. They are not cheap normally, but probably best to just go get one you can hold in your hand and see that it fits.
Many of the senior dials have ears with four slots for bolts to drop in but many or most have the two screws so the dial can be removed from the 4 prong adapter and put into your gauge.
I think that would be the right thing, too, if you can get it. The one in the picture is Rochester part number 5-01023. Maybe call them if you can't get it from Cat. That is if the Cat markings are right.
http://www.rochestergauges.com/
