This is one of the rarer Third Ave. photos. Car 551 was the prototype Huffliner and the only Huffliner to receive a trolley pole until they were all sold to Sao Paolo, Brazil in 1947 and converted to single-end overhead operation.
The car was built as a sample by Brill in 1936 and tested (here) at Garden Avenue yard in Mt Vernon prior to being retrucked and towed south to Manhattan for service. Unlike the production model Huffliners built in Manhattan, this car had the center doors opposite one-another.
Courtesy the Bill Volkmer collection