At least that's what worked in my set up. I made two trips to the hardware store today, because my admittedly brief search on the subject turned up no hard data. 1/2" 20. Fortunately the center rack is a cheaper knock off, and as such is not threaded at those holes (which are placed at the correct points). This made bolting the three racks together easy, once I had the right part.
I've also had the opportunity to assess condition/damage on a few long neglected devices. The beat up RCA BC3C has shot into total restore/rebuild territory. Ugh, ugly. Mice. Gives me a focal point to hinge my activities this winter.
It'll need more than a tweak on the hum balance pot, that's for sure.