While I can only speculate about the purpose of the reduced load, I do have a normal PGU-15/B TP round with a purple case made by IVI in 1975 and loaded by Aerojet in 1976. The markings are almost identical to those on your friend’s case, differing only in dates and lot numbers, and of course lacking the “REDUCED LOAD EDD 737” marking.
I assume that IVI was simply a supplier of shell casings during that time frame.
There are a couple of these reduced load rounds at http://www.destructivedevice.com/ordnance.htm, 4th picture (they look like fired cases with TP projectiles inserted). One has a purple case similar to your friend’s, and the other has an orange case manufactured by ACN (Amron), loaded by Aerojet.
I’ve read that during initial testing of the GAU-8/A-10 there were some flame-outs resulting from the engines ingesting gun exhaust. Perhaps the reduced loading was an attempt to address this problem? This is pure speculation on my part.
According to my references, the initial in-flight testing of the GAU-8 happened in 1974 and the gun was officially adopted in 1977. Thus 1975-1976 would date to the testing phase. I have a couple of dummies and the above-mentioned TP round that date from 1975-1976.