GMCo appears to be more mysterious than I thought, especially for
such an evidently long-lived and versatile company (?) A little
more googling amidst google books shows three addresses:
1308 43rd Ave LIC (late 40s and early 50s), 43-10 21st St. LIC
(1960, 1969), and 180 13th St. LIC (1929 !). Guessing from the
google snippet it looks like they were selling shock absorbers in
1929, tire-valve caps in 1963, the "Watchdog" anti-car-theft device
in 1969, sharpening stones in 1942, alongside my little vise, a
small lathe, the drill adapter mentioned above (ads, 1959-1963),
and a number of equally various things.