Did the guy not recognize the voice at least? Cassidy had been on The Addams Family for two years by that point. Or was his Lurch voice that much of an affectation?
Surely it was.
Oh, I’m sure he overdid the gravellyness of it, but I think it would be close enough to his regular voice to be recognizable. I’ve seen him in other roles and his voice is still as low as a foghorn.
“Rules? In a knife fight? No rules!”
In the 1960s, there were likely some people who hadn’t gotten that much into TV yet. Or, they had another fave show on at the same time and they didn’t have a VCR to time shift with, being the 60s and all. Or, he was a stereotypical Ins salesman and wasn’t really listening or noticing much of anything outside of his sales spiel.
Preceding my favorite scene in* Butch Cassidy and the Sundance Kid*. 
…after which Ted Cassidy spoke much higher.
I believe the moniker was Cut Nose Curry.
Ah, thanks. Guess I misremembered.