I agree that the headers and collector box are possibly custom built. Cylinder three's pipe has been bent to be able to access the oil filter without having to drop the headers, which was a common issue with aftermarket systems of the time. There was a company in the 70's that introduced this concept - but sorry, can't remember their name.
There was one made by Triple A that looked all bent like that, but that's not it.
May 1972 Cycle magazine did a test of a bunch of CB750 period exhaust systems.
article was "pipes galore for the Honda 750"
Hindle have built exhaust for Hondas for decades, they still make pipes for vintage bikes. If you don't want to spend new price you may be able to find a used one. Prices are in Cdn $, take 25% off to get a US $ number.
Looks like the S&S I had before it rotted out (British company not USA)
The two different lengths were deliberate and actually worked real well but I forget which magnitude exhaust pulses they were tuned for
I'm curious just for the ease of maintenance. I do like the Delkevic pipes as well and I see many people like them. I'm possibly thinking about trying a 4-4 set up with some 18" reverse cone style mufflers to keep a bit of my bikes vintage look. The originals are bit large for my taste. Although I'm still a ways out from that part of the build.