Those are fun machines, made between 1982 and 1985 probably. The placard on the side doesn’t have a date?
The button is possibly a payout button, have you tried adding credits and then pressing the button? Or play a game, win some credits and then press the button? Usually there is a placard around the button.
In general the machines are pretty basic, single game video poker. They are robust and the boards can be repaired if they ever fail, K-Las has a testing station for them.
If everything works they sell for $350-$450 but you will probably have to wait for a buyer who has interest in early video poker.
Chris