i'm pretty sure every small block and big block had EGR valves by 1973. properly functioning, it's actually a good thing that can help you live with too much compression with crappy gas since it dillutes the incoming air/fuel mixture with exhaust gasses under part throttle conditions, which can keep it from detonating... but they never work properly for very long, so it's best to just get rid of it and block it off..
regarding the carb: it's an aftermarket non emissions compliant part, so they have to put that disclaimer there..
Back in the day I had many cars come into my shop with the complaint that the car was pinging. A test drive always confirmed that the customer was hearing correctly. Typically I would find an 1/8" BB inserted in the vacuum hose to the EGR valve. Removing that BB would instantly cure the ping issue. Friendly mechanics were doing that as a favor to their customers who hated those government mandated pollution devices.
This was common in the late 70's. In those early years when the manufacturers were trying to deal with the pollution edicts from our benevolent government they developed the EGR to help the engines then being manufactured deal with the ever leaner fuel air mixtures.
For all practical purposes an engines exhaust gas can be considered an "inert gas", meaning it will not support combustion. The EGR ports a bit of that exhaust gas into the intake stream so as to "cool" the combustion process, which was leaner and hotter than in earlier non-epa years, and eliminate the characteristic ping that came along with those leaner mixtures.
Additionally, as the years went on the EGR and its associated exhaust passage would become clogged with carbon deposits and cease to function even though the valve itself was still functional. Cleaning out the exhaust passage and the EGR valve with a pick and a vacuum cleaner would get rid of the ping once again.
