There is not a universal key for pinball backboxes like there are for jukeboxes and other coin operated devices. (The coin boxes on those have unique keys but many models shared the same keys to the guts, making things easier for service people)
If I understand correctly, the lock is unlocked so it is not preventing you access. If you do decide you want a key, as mentioned before, if you take the lock to a locksmith he can cut you a new key for it. I would add that it doesn’t have to be expensive, they are really simple locks. My local guy only charges about $10/$15 to do it if I leave it with him.
Since it doesn’t have to be Fort Knox quality, you can also disassemble the lock, slide another key in the exposed cylinder (maybe your coin door key?) and file down the proud brass bits flush with the cylinder until it rotated freely in the barrel with the key inserted. Now you have both locks keyed alike and it will be secure enough for home use.

if you have access to a 1382 or 7500 key, those are the most common keys used recently and the closest thing to a universal key these days (only if the game has had new locks in the last 10-15 years or however long Steve Young has sold those 1382 locks)

I have several coffee cans full of old pinball keys sorted by type that have never let me down when I get a game without keys. (not applicable to you but for the sake of others - picking the lock and installing a new one is a lot faster than going through the can!)

I've worked on several hundreds of games myself and I haven't run across the "limited" key unless a bunch of games in a certain geographical area were all keyed alike by an operator at some point, so it is very possible.