"The Cooey model 84/840 [shotguns] that were manufactured before 1961 are stamped "H. W. Cooey Machine & Arms Company" on the right side of the action and the guns made after the purchase from Winchester of Cooey are stamped "Winchester-Western (Canada) Limited." on the right side of the receiver. Written records of serial numbers are not generally available. Some have suggested that Cooey did not use serial numbers, but many pre-1961 model 84s are stamped with serial numbers on the receiver, barrel (under the fore stock) and under the butt plate/shoulder stock. Dating the serial numbers remains a challenge for Cooey gun collectors." -Wikipedia
So according to this, its pre-1961, not much different from what I figured already. There's a couple different conflicting statements in the Wiki article that says 1961 or 1967 for the Winchester takeover