The original Atlas drawings for their 7/8" and 1" arbors both show key slots.?? I am in the (probably bad) habit of not using a key an my OEM 7/8" arbor and, so far, have gotten away with it.? I made my own 1" arbor and did not include a slot.? I was worried that adding the slot might introduce runout.? So far this has worked out, but I might regret it one day.
My Atlas owner's manual from 1948 shows a "wiper" but no frame.? The wiper is just rectangular piece of material, maybe felt.
A lot mods and improvements for Atlas Mills appeared in Popular Mechanics in the 1950s and 1960s.? The frame might have been from one of those articles.
Jon M