My books on Brownings are boxed up at the moment,so I will have to use other sources:
My old Blue Book states that from 1958-1976 Brownings were serial numbered with a number first followed by a letter denoting type of gun.
So the "8" would be for 1958 (most likely) or 1968 (probably should be a 68 instead).This must have started sometime late in 1958 (in honor of my birth perhaps?
The z would probably denote a Lightweight 20 ga frame.
* Full choke (I have read all the barrels start as full choke and are opened to whatever is wanted)
*- (or one half star) Improved Modified
**-(or one half star) Improved Cylinder
Hope this is correct and helps you.