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.