The 19 Jr. has a full size arm board made from partial board and has Sorbothane pucks as the suspension.
With the HW-19 MKIII, the sub chassis changes from one inch thick MDF to a non-magnetic, 10 gauge, stainless steel sheet laminated to a 1/2" shiny black acrylic top plate and tonearm board with a spring suspension and a cork covered bottom platter, with lead inserts. The tonearm board is removable for easy arm changes, but is fastened to the steel sheet also.
I have had both and like the JR. with a MKIII platter and the Sorbothane pucks replaced with Herbies Tall Tenderfeet better.