Did Peter Brock really design the IRS? As far as I know, it was developed at Ford by Klaus Arning and co.
"One of the cars will have a 9-inch fixed axle rear end. The other two will feature the pioneering Peter Brock-penned independent rear suspension system. Brock designed the IRS setup for the 1965 season, but the car won without it and it was not put into production at that time."
There are 3 IRS designs. Dwane Carling has the blue prints of the Ford design which I believe originally used a Salsbury (Cobra/Jaguar) pumpkin and is the Arnig design. That one actually had pin drive wheels designed for it that were held on with knock-off spinners designed to go with it simply by replacing the Mustang hubs.
He was selling copies of those and I think I bought a set and have them here? But where? Hum?
There is another that uses a shortened Mustang/Falcon banjo and 9" Ford pumpkin. I think that's the Brock design? I know that the "continuation R-models" have an add on option for the IRS. Carling may be supplying those with the Ford pumpkin design. You need to inquire with Brock about that.
There is mention of the Green Hornet also having an IRS which Fred Godel said he designed for the car. That one was not on the car when it was discovered on a used car lot in Michigan and to the best of my knowledge that IRS was never recovered or the blue prints found, but that might be yesterdays news?
Vinman had a stack of blue prints from Ford I think at the NJ "Playboy Club" Convention and although I don't remember seeing them, could have been in that batch? I don't remember. I haven't thought about that in 30 years or so?
They are all similar in that they were intended to use the Mustang "pick up points" (spots where nut certs were installed in the unibody to mount the rear springs) so that they could be just bolted in, in place of the rear live axle and leaf springs.
They are an interesting piece of history but as you said, Brock was disappointed that the cars weren't faster by even a second with them installed.
Not having driven a car with them installed, I would have to think that the car likely rides better since the live axles can jounce the car around a bit over dips in the pavements. That kind of adds excitement at 130 mph though.