Gates claimed that the whole Super Mag series was developed in October/November 1975, except for the necked down variants and the lengthened straight wall versions. Obviously, the .460 S&W Magnum didn’t exist then, and I’m not entirely certain that the current length .454 Casull case even existed then. I believe Dick Casull was still using standard .45 Colt brass at that time for his custom revolver conversions.
Gates had a huge grudge against Freedom Arms, and often wrote dismissively of their revolvers and the .454 Casull cartridge. It seems that he had approached them to offer a special discounted deal for IHMSA members, and they rebuffed him as his asking price was too low. In return, Gates ramrodded a price ceiling which effectively outlawed the Freedom Arms revolvers in the Production divisions. Until the rule was rescinded years later, those who wanted to compete with the Freedom Arms had to do so in the Unlimited division against customized T/C Contenders and Remington XP100.
The .505 Super Mag was supposedly built off of .470 Nitro Express brass, while the .500 Linebaugh series was made from .348 Winchester brass. The .610 Super mag was reportedly made from .577 Nitro Express brass. I don’t think anyone has tried to stuff that into a revolver yet, but J.D. Jones has a version made from full-length .577 Nitro Express brass called the .600/577 JDJ, and another variant with a 2" long case called the .620 JDJ.