I recently realized that after getting no help from Hank's Truck Pictures, I had no other choice but to make a last-ditch effort to identify an NYC Sanitiation truck in "Up the Down Staircase," by writing to the Department of Sanitiation itself.

http://www.imcdb.org/vehicle_180522-unknown.html

And I just got official word from the Department of Sanitation, and I quote;
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"Great day Mr. Dey and fortunately we are able to make a reference to one of our vehicles compared to the one listed in the website that you provided; DSNY has a vintage Alley Collection. The manufacturer is IHC (Esclator Truck), Model Year 1962, Fuel Type Diesel, and is currently used as our museum truck. I hope that this information assist you.



Deputy Chief Keith Mellis, Executive Officer
Public Information and Community Affairs
Saturday, January 10, 2009"
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I don't know what an "Esclator Truck" is, but this clearly looks like an R-Series.

One more obstacle solved. Now, I want to find this truck.