The pistols have a small diameter "spring" that inserts (very short leg) into a hole in the frame on the right side, inside the frame (you can see it with the slide off). The long leg of the spring goes under a pin in the frame (right side of the frame, again) and puts pressure down into a slot/groove in the top of the slide stop shaft. When you remove the slide stop you can see a little groove where the flattened end meets the round shaft.
The right side slide release uses that same spring to hold it in the frame. The hollow section/shaft of the right side lever fits over the left side slide release shaft but has a slot cut in the top of it. That slot allows the spring to set down through the hollow shaft of the right side lever to fit into the groove in the left side lever and hold both levers in place.
The left side lever is removed by lining up the notches in the slide/frame on the left side and pushing it out with something (I used the hard plastic magazine base/floor plate) so your can remove the slide/barrel/recoils spring and clean/inspect/lube.
The right side lever can be removed, if you feel the need) by rotating it around to the point where the spring is forced out of the slot in the hollow shaft and rides up onto the solid portion of the shaft. Then you can just pull the right side lever out of the frame as the spring isn't holding it in place any longer. The right side lever can be a pain to get pack in as it isn't beveled on the end to make it pop past that spring easier.
When I rebuilt my Pre B CZ85 I ordered new left and right side slide release levers. I talked to CZ Custom and was told that sometime in the past the right side lever was upgraded due to breakage of the hollow shaft. The metal was too thin and some were breaking/cracking. The fix was to make the hollow shaft walls thicker (same ID, larger OD) which required the hole in the frame to be larger. I elected to stick with the original at the time (having done my CZ85 Compact clone project, now I'd go ahead and drill the frame out myself, but at the time I just didn't want to do that). If mine ever breaks I'll have to make the frame hole larger.
The newer style right side lever will work fine with the older style left side lever and they didn't change those, they are still the same piece (if I understand what I think I heard while talking with CZ Custom).
If the pistol you're looking at had a right side lever and it is now missing you'll be able to tell as there will be a big gap between the frame hole and the left side lever shaft due to the missing right side lever.
Good luck with it.
Oh, couple of other things about some of the Pre B pistols. Magazines - not all Pre B's can use modern magazines. There are some threads on the forum here about modifying the internal areas of the mag. well to allow modern magazines to insert just fine. Yours, being a transition model, may not have that issue.
Another thing is the barrels. Again, a transition model may not have the issue, but my Pre B would not accept a modern replacement barrel. The barrel and slide were made different. I had to open up the inside (near the front of the rails - it's all internal so you wouldn't even realize it - been refinished, too) of the slide near the muzzle end to allow a modern barrel to fit/function. My original barrel was pitted pretty badly in the chamber and barrel sections so I figured I'd better replace it.
I think the Pre B's are neat. I wouldn't want a CZ75 Pre B as some of them are impossible to get safeties/parts for the safeties for these days. But the 85's use all the parts the newer 85's use except that right side safety lever (well, all the parts but the front sight and the barrel bushing, but that's another story).
Again, good luck with it. I like mine.
