I was curious after reading this thread so I grabbed my 1951 vacuum grip pliers and they (surprise, surprise) had a conventional nut on them. They came right apart and the thread on mine seemed to be 1/4-28. I have quite a few Crescent made pliers so I took a pair of Millers Falls (by Crescent) and a pair of Cee Tee (by Crescent) apart and they both had nuts that were the style you are looking for and they are 1/4-28 as well. After successfully installing both nuts, I put my SO pliers back together with the dark nut because I liked the idea of the field repair. If you would like a pair of the Cee Tee pliers with a nut that removes easily, send me a PM.
-Don
My pliers are model 47B so yours may be different.