My 73 is a factory A/C car and the inlet/outlet hose nipples do not look like that at all. the simply come through the firewall with no visible mounting brackets. and I don't believe the factory used that kind of goop to seal the firewall, only gaskets.
Only difference is I converted mine to new compressor and 134.
One other note, if it were a factory big block A/C car it would have a 28 inch radiator which would also be on the fender tag, last item just says '28'.
A/C would be on the build sheet if you have it as well.
My guess is it was (and nicely done I might add) either a dealer add on or someone did it. But as I said my guess ...
Beautiful SE indeed.
Mine:
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