I did a serch on the internet and found the studs being used are mostly homemade from Grade8 all thread size is 5/8" x 11 TPI, then just cut the all tread to fit your needs.
The plates I have came with my 60 when I bought it. They are cast and looks like the type that had the swaybar connected to them...I don't know exacty were they were used. My guess is 1Ton 73up Truck?