In my last job we had a nice old cash register made in 1986. One morning I came in and heard a very sickly sounding beeper. As I came closer to the cash desk the smell of cooking electronics became stronger until I could eventually see a constant stream of smoke rising from the cash register.
The poor thing was dying and crying for help.
I pulled the plug and after some head scratching decided to open it up. It used a 6502 processor and a 7 pin matrix printer. The noises this thing made when operating were brilliant. Turned out some caps on the PSU PCB had given up after >25 years. Problem was I really needed the register for daily business and so decided to try and fix it - after all this was an electronics shop I was working in.
This actually worked and the thing booted up after this but as it turned out it had forgotten its configuration. The user manual was of no use to me, I couldn't figure out how to program this old beast so had to call the service tech to re-programm it.
He did it from memory, took him about an hour of keying in arcane codes and the cash register worked again. He told me we were the last ones using this ancient model and he hadn't seen another in service for at least five years. I was impressed that he remembered how to set it up - he's probably one of the better service techs.
All cash registers I've seen so far have that release catch for the drawer on the bottom. You have to be able to get at the money even without power and keys.
The key switch is the mode selector, P is Program mode, X is used for things like cashing up at the end of the day, Z is normal register mode. Have forgotten what the others do but they're usually the same on all cash registers. The keys used for these locks are quite nifty, there are usually at least three different keys, each key performs another function. So there's the P key to get into P mode, then there usually are user keys for the people operating the machine and superuser keys for the boss, for things like cancellations and sub-totals.