First production was more likely in 1958, based on Sears catalogs and their probable printing schecules. The Atlas 3991 (Craftsman 101.28970) was produced up into 1967 when it was replaced by the 3996 (Craftsman 101.28990). The only change known was that the pull-knob power cross-feed was replaced by a lever. Except for the knob instead of lever, your photos could be of my 3996, even to the heavier aftermarket 6" 3-jaw chuck. Very nice looking rig.
The casting date of the bed is in raised numerals on the back of the bed. If you can read it. Otherwise, the only other date actually on the machine is engraved into the two spindle bearings. But you have to pull the spindle to read them.
Robert D.