I have the Sears Power Tools catalogs from 1932 through 1981 and to check, just looked through several between 1942 and 1957 which spans the years that the AA or 109 lathes were sold. It doesn't appear that Sears ever sold a steady rest for them. And as Pat said, the beds are unique. So any steady rest you can find to buy will have to be modified to fit the bed. Doable but not trivial.