[DaleHCook]

Next photo is the other end of the speaker cable and has a 4 pin plug with a round pin position locator and the third photo of the receptacle mounted on the PS/Amp chassis.

That is a pre-WWII plug and socket made by Howard B. Jones, which was later part of Cinch-Jones. I know it is pre-WWII because of the round cap on the plug. By 1942 Jones was using only rectangular caps (at least, that is all I have seen in catalogs). They were still making the same 4-pin plugs and sockets with the polarizing pin in 1942. Also, MrFixr55 discussed the use of the Jones plugs on earlier Victor models.

I have good illos of the R-35 / R-39 / RE-57 speaker chassis in the 1929-1930 Red Book (all three models used the same chassis), along with a full parts list. The R-39 chassis did not use a Jones plug (which is shown in the 1929-1930 Red Book for the RE-45).

As far as I can tell the speaker chassis in the R-35 / R-39 / RE-57 is unique. I am leaning toward the suggestion of MrFixr55 that the cable was soldered to lugs. I will probably have to wait until I have the chance to get at my R-39 in storage to find out.