Inspired by the 1979 Mississauga train derailment and subsequent evacuation it seems to be more of a novelty record than something done by a regular band. Neither of the 2 versions I have has a date but it would presumably be from late 1979 or early 1980 in order to quickly cash in on the event.
I have no idea if both sleeves were done at the same time. Perhaps the more topical "Evacuate Mississauga" one was done first and then the "Special This Week" came later once the event had passed and there were unsold records. As far as I can tell the 2 records are identical (matrix numbers are C-301-A-1A-2HDZ / C-301-B-1A-2HZ:). The label number is P.W.K.R. C-301 while the back of the sleeve lists Bomb 5020.
I've heard as few as 100 or 200 were pressed but I have no definite number. With 2 different sleeves I would have expected a larger number. It does seem to be a fairly scarce item these days though.
Gas - Evacuate / Special This Week 7" (P.W.K.R. C-301 / Bomb 5020, 1979?)