A chip should never erase no matter how long it sits.
The little window on the top of the chip is however for erasing and that is one reason that people put a label on the chip. Of course labeling a chip is a good idea as they all look the same otherwise.
To erase the chip you typically need a Eprom eraser which is a high powered UVlight and you typically need 10-15min.
Chips can however go bad especially if they have been erased and re-programed a number of times. Given that there is no label on yours I will surmise that this is not a factory IGT chip, and someone burned this eprom for you which likely means that this was a used chip. They are not making these chips anymore so a good number are re-used.
EEprom (electrical erase) which yours is not can be erased in a few seconds, but again you need a Eprom programmer that can apply power to the correct pin.
Super glad that Jim figured this out for you.