There are in fact 1/4-28" thread parts on the older Radio King era Slingerland castings. Those I've encountered were on hoop clamps used for tom mounts, cymbal holders, spurs, etc. At least 50% of those wingscrews still in circulation are badly worn due to overtightening and poor storage conditions. Usually the female thread castings are still usable. I was once informed that 1/4-28" threading was somewhat common mid-20th century and earlier but that it fell from favor. It might be possible to have a good tool and die maker construct some round or hex dies for you in that size, but otherwise those old Slingerland parts are hard to find.
However, as for the tension rods used for all older vintage Slingerland drums I've never seen anything other than the standard 12-24" threaded rods. There are many older drums (Leedy, Duplex, Gretsch) made with 10-24" tension rods but I've yet to find Slingerlands with them. Without actually seeing your drums in person this info is the best I can offer at present.
I suggest purchasing pocket thread gauges from a hardware dealer and eliminate guesswork. That will save you a great deal of time and trouble.