I just bought a beautiful 4-page 1981 Infinity ad brochure from eBay, featuring RS-IIIa, 4, 5, and 6 speakers. It has a couple of nice photos of the speakers, but moreover this is an old school ad/brochure where the selling is done via talking tech and physics. In other words, lots of good info on the design principles and physics behind these speakers. Below a PDF scan of the brochure. I had to put it in two parts due to size limitations, sorry.