One of my shooting buddies has an oddball G20. It is completely OD green. He purchased it NIB in 1990-1991 and it was so ugly in his opinion he never shot it.
I just became aware he had it, I had not realized tha they (Glock) had made OD guns back then. IF anyone has any info on this it would be appreciated.
To my knowledge, Glock has never done an entirely green pistol. The OD frames were only available since the 3rd generation came out.
This could have been a limited run for a distributor like Lew Horton or something like that. Ask to see the box, their could be clues on the label.
Are you sure it wasn’t just unfired in the box? Like someone bought it, had it coated/painted then traded for something else?
I will try and get pictures tomorrow, he is supposed to bring to the shop.
Get the serial prefix(i.e. AAA***US) too, that might help. 
Standing by for more info and/or pictures.
I am standing by, waiting for it to appear as well. I am still bugging him though, so as soon as I see it, I will have pics.