I don't think all plants used that type, I know mine was original, there is even a special end on the vacum line that hooks up to it that will not work with the pancake one. I have only seen a few other A/C Chevelles like that.
In any case I bought mine from a place in TX 4 yrs ago, at the time they had two left. Just for the heck of it I called them, they still had the other one, the last one. Needless to say I bought it, paid 65.00 - will keep it as a spare.
I'll try and remember to snap a pic and post it with the screws. You can just make them out here - can copy this pick to your p.c. and blow it up - may see it better.