I have one of those from new.......it was great until the seals went out on me and wore the retainer to hold the pump handle closed.
There's some information on those online but from what I remember they started making those around 1977 and think they chanced the model around 1981. The best one's are the metal breech (early models). That was the change and how you can tell and early model as the newer ones had plastic breeches.
There's a few parts dealers for them if you do a search, the one I remember is Airgunsmith. I need to rebuild mine too but it's a project for another day. There's also very few diagrams of this online but it's been a while since I've done a search. If you pull it apart be sure to photo every step as you go. I have a couple diagrams somewhere that I found but sure due to legalities can't post here. Look around and they'll pop up for you.
Here is the one (1) WARNING they should have actually put on it in big BOLD lettering........once you start using it you will understand. Be careful to keep the pad parts of your hand away when pumping. I think there are still pinch marks on my hands from 39 years ago where I pinched my hand from pumping that thing. LOL you'll see...........
Make sure you keep the pump oiled. There is a felt patch that is exposed when you open the pump handle all the way. This patch needs pellgun oil regularly so things don't dry out. I'll be interested to watch this post as I loved mine and want to get it up and running again. Many a bird fell, along with leaving a few welts on the local cats sneaking around the yard. 10 for kill birds, 2 or 3 to let a cat know they weren't welcome!