The Model D was listed for only 12 months during 1936-37 (with a simplified version  - the Model "E" - as part of the Atlas "Unit Plan" system) before the announcement of the power cross feed Model F which had not only a completely redesigned apron but a leadscrew increased in diameter from 5/8" to 3/4". Just before this the opportunity was taken to significantly improve the top slide assembly the base casting of which had originally been in ZAMAK, a material unsuited to the strains of being fastened by two T bolts into a circular T slot cut into the top face of the cross slide. As the base casting was tightened down - and almost certainly exacerbated by taking heavy cuts - the casting distorted and caused the slide to bind. Atlas solved the problem by casting a stout post integral with the cross slide over which the top slide dropped. Two square-headed bolts (the front one can be seen in the illustration above), passed though through the lower of the two top slide castings and pushed short bevelled-ended bars against an inverted conical face on the post and so drew the assembly down tightly. With just the post being squeezed the distortion to the cross slide was eliminated.
Note the bed feet: the model immediately before this was without the inward splay to the castings.