Atlas 6" x 18"  (3" centre height) - 1941 version with the taller, Mk. 2 countershaft with the swing head hanging down backwards and the V belt passing between the uprights.

Continued:
During the early 1940s, as industry switched over to war production, several variations on the standard lathe were produced including Craftsman-badged versions built to the better Atlas specification with two different types of roller-bearing headstock (instead of plain bearings); one was a standard Atlas unit with just a badge change and the other a specially-made headstock that resembled (with its frontal bulge) the shape of the ordinary Craftsman plain-bearing unit. For more details of the Craftsman versions, including departures from the official specification, click here (if any owner has an original, stand-mounted 6-inch Atlas or Craftsman lathe, or a parts list dated as being from the 1930s or 1940s, the author would appreciate hearing from them).
Tumble reverse was fitted to the changewheel drive so allowing the spindle to run freely or to cut right and left-hand threads by just lift or lowering a lever into spring-plunger indented positions. Because there was no shear-pin incorporated in the drive to protect the ZAMAK changewheels when the carriage ran into the headstock under power late-model machines (from some unspecified date) had plastic "safety" gears fitted to the tumble-reverse studs. Changewheels on all versions of the 6-inch were 24DP and with a 14.5-degree pressure angle.
A simple but adequate single-sided apron with proper twin clasp nuts and a "geared-down" drive from the handwheel to the leadscrew was used and remained unchanged for the life of the machine. Like that used on all other Atlas lathes the cross slide was of the "short" type that tended to wear the ways in their middle section, though it did carry a sheet metal cover at the rear to protect the feed screw. The top slide could be rotated through 360º and, happily,  was T-slotted - so easing the fitment of alternative toolposts to the simple and traditional single holder fitted as standard. Unfortunately both the feed screw micrometer dials and handles were tiny though by way of compensation all the carriage hand controls, even the leadscrew clasp-nut handle, were chrome plated.
The leadscrew was threaded 0.5" Acme and a dial-thread indicator included in the standard equipment.
Although the headstock carried a No. 2 Morse-taper centre, the tailstock was only a No. 1 - possibly, together with the tiny feed-screw micrometer dials, the only specification failings in an otherwise well-designed little lathe. A shorter-bed version, the "Model 612", with just 12 inches between centres, was also manufactured - but this seems to be a comparatively rare machine, despite the undeniable attraction of being light enough to be lifted by one person from under the stairs to the workbench.
Although they are very different, confusion has arisen in the past between the Atlas/Craftsman 6-inch, and two other lathes sold be Sears (but made by the AA Company) the "109" and the later "Craftsman 80". The lathes produced by AA were very simple, minimalist machines and, in comparison with the Atlas, very inexpensive; in the mid 1950s an AA Craftsman 80 cost $48 - compared to over $160 for the Atlas.
The lathe was copied in post-WW2 years in Sweden and sold as the Högbo - though a version of the early countershaft was used, the bed was longer and given a V-way at the front--and there were probably other detailed changes as well
In the early 1970s Atlas introduced a Mk. 2 version of the lathe; this was styled in a very "square" fashion and, with the traditional separate speed-reducing countershaft removed, had its 8-speed drive taken directly from the motor by a tensioned V-belt to a multi-step pulley mounted outboard of the left-hand end of the headstock spindle; details of this lathe can be found here.
More on the Mk. 1 Atlas 6-inch: page 2  page 3

Roller-bearing headstock with flip-top oilers - adequate for this model, but not for the plain bearing of the "Craftsman" specification model which would have been safer with proper, continuous-drip oilers - or restricted to owners with a good memory and a conscientious character.

Early bench-type countershaft unit used from the lathe's introduction until some point in 1940 when a modified version of the more robust stand-type was used instead.

The carriage assembly - with its 360 degree rotating top slide - remained unchanged during the machine's production life. Even though a T-slotted cross slide would have greatly expanded the lathe's versatility, Atlas never offered one.

Simple but adequate single-sided apron with proper twin clasp nuts (in ZAMAK) and a "geared-down" drive from the handwheel to the leadscrew. Additional carriage pictures can be found here.

Detail of carriage drive gears, carried on a ZAMAK housing,  that gave a reduction between apron handwheel and bed-mounted rack.
More Atlas 6-inch pictures: page 2  page 3