This model was originally requisitioned by Atlas as a replacement/clone for their old A1G 40' composite box car (the one originally made for Atlas by Rivarossi). This Mehano knock-off was subsequently sold by MRC and Life-Like (and with the later production having somewhat improved shell detailing).
Revised version -
I'm not sure what specific prototype these models might be based on (if any), but going by the number of side panels (six) and the trusses next to the doors (which slope away from the doors rather than towards them), the prototype is likely an ACF-designed "Howe Truss" box car.
(Thanks for the pictures, Alan)