The model 840 was made between 1967-1979. I know of no detailed production records to narrow that down.
I own a Cooey Model 84, serial number 7690 estimated to be manufactured circa 1950. Cooey began making shotguns in 1948.
Cooey model 15561
what year was model 4b serial number cao86767 made
The serial number is on the passenger side of the receiver, just rear of the barrel. If there is no serial number your cooey was manufactured before 1968, when no s.n. was required.
150 - 200 bucks in .410
Model 75 was made from 1931-1961 by Cooey (Canada) and then bought out by Winchester who made them as Model 750s until 1978. If you have no serial number that is due to the change in the Manufacturing laws. 1968 and after should have serial numbers.
By the serial number on the rifle.
Could be from the 1960s, likely worth no more than $100. Try consulting your local sporting goods store.
Numrich gun parts
If there is no serial number it, it was made between 1903 and 1961