I'm going to guess that with the introduction of the self-contained 302 sets in 1937, this type of jack went into use. For all stands produced prior to the advent of the 302, a subset was required and it likely functioned as the jack.
I don't know if jack covers were ever made of bakelite, but if they were I'm betting they didn't get along well with upright vacuum cleaners of the day.
I did some checking and there were mounting plates for the subsets, but most subsets found at swap meets etc. today don't have them. It seems the plates often remained behind when the subsets were removed, or were otherwise separated somewhere along the line.