Chevrolet Blazer, 1970, w/o AC.
model# 2-275
(It'll fit for 1969 through 1972)
It's supposed to be 9 1/2 inches by 6 1/8 (but that may not include the tanks), with a 3/4 inlet and a 5/8 outlet.
It will also fit as the rear heater of 1973-1975 Suburban, Tahoe and Yukon, or as the main heater of the same models, w/o AC, 1967-1972.