Neil - Unless those brackets are part of the original knee action hydraulic shocks, I doubt they are original. My '39 chevy 1/2 ton chevy came with an aftermarket overload spring arrangement on top of the axle. Could these brackets be the aftermath of something like that?
Are there any spots on the frame where overloads may have rubbed at one time? I'm not sure that I have the correct torque tube in my '39. It seems too wide. Could you do me a favor and measure your rear axle from backing plate to backing plate or from the outside of the drums? I'm about 60" which is 2" wider than my front. That'll give me a reference point. If its not original I may go to an open driveline. Thanks