What year are these 2 belgium made browning guns 20 gauge 8Z9039 and light 12 gauge 31920?

Lets start with the 20 gauge model.The prefix of 8Z to your serial number indicates that your Browning light weight auto-5 shotgun was made in Belgium in the year 1958.The light weight 12 gauge was made from 1953-1976.All light weight auto-5,12 gauge shotguns had a prefix of the letter L,or the letter G from 1953-1958.After Words the prefix had a combination of a number and a letter from 1959-1976.This would appear as 8G,or 72G etc.Please check your light weight model auto-5 shotgun for this,and then we can identify the year of manufacture.