with slightly worn Type II trucks, this car looks to be from the early 1920's so The Joy Line production was still a few years off when this set was made. i assume the other coupler on this car is the standard "garter-loop" type that is tabbed into the frame as a sliding coupler? my guess is that the cars were adapted to run with a different loco/ tender and other couplers types are sometimes too large for the narrow slot in the Flyer coupler.
coupler height sometimes comes into play, but a large assortment of prewar cars from different manufacturers are very compatible with each other.