The first edition of Aftermath (1980) was produced by Phoenix Games. The few of these early Phoenix sets that I’ve seen have Fantasy Games Unlimited stickers over the PG company logo.
The lid art is different from the FGU sets that would follow: The man is drawn slightly differently, and the woman is missing. The contents are identical to the later (1981) edition.
I’ve seen plenty of examples of an alternate 1981 version with white-covered books (as opposed to the more usual yellow covers).
Phoenix Games 
FGU Yellow books 
FGU White books
I asked Scott Bizar (FGU owner) about the white title background versus yellow. Which came first?
The yellow books were from the first two printings (at least). White cover stock was used later. Remember that we reprinted AFTERMATH! every year so there were a good number of printings and I don’t remember exactly when the switch was made.
Later FGU published a combined Paperback edition, incorporating all the books from the box set.
Aftermath! Roleplaying Game
1980, 1981 … Robert Charrette & Paul Hume … Phoenix Games / Fantasy Games Unlimited (FGU) 4000
Aftermath RPG items in the shop
The Aftermath RPG product line cataloged at my reference site
EDIT May 24, 2020
Aftermath RPG items in the shop
See also:
CITY STATE: A drop-in Chicago Sourcebook for your T2000 campaign
Early print of Aftermath! Magic!