Mike Holt's forum, discussing what breakers will fit in that box.
If you remove one and inspect the bus, you'll see the attachment portion of the bus for the THQP breakers is perpendicular to the main stabs that the type THQL breakers attach to. THQP breakers snap around the bus where the full sized THQL breakers fit tightly over the bus. This is obvious if you inspect the breakers and bus carefully. If you remove enough breakers, you'll see the difference in the bus in the areas that the full size THQL breakers MUST be used and won't accept the thin breakers. At this point, you'll have to use your head to determine which spaces will accept the thin breakers. The only other recommendation I could make would be to go to a supply house that stocks GE load-centers and ask them to explain it to you and show you the difference.
The breakers with the green labels look the newest, take one of these to the store with you and make sure it matches the mounting. I'd call it an 8 slot panel, using tandem breakers to make 12 branch circuits (otherwise there'd be only be 6) and one (2 pole) main. In your panel, it would have been borderline acceptable to take the cover off, say "ewww" and put it back on, having TOUCHED NOTHING. Now that you have went in there and played with stuff, I'd be concerned that you have loosened the connections of the wires and/or the buss-bar contact clips of the 50yo breakers. If you're going to continue onwards with that panel, consider getting those leads away from the neutral bar, shortening them while you're at it. At least check ALL of the connections. Upgrade that panel, please (or KEEP OUT). I don't want to see this in your next post: