The M66 was a product of the late 1950s through the late 1970s. I think they were finally phased out in the early 1980s.
They are functional, no frills utility shotguns.
I have one in 20 gauge that was my Father's. Kicks like a frigging mule.
Yours is a relatively uncommon slug gun, as denoted by the recoil pad and the iron sights. They are worth probably between $150 and $175 for one in good condition.
If you've not fired with it 3" slugs, when you do...
Hold on. You're in for one hell of a ride.