That's great you used to dive one! fun helmet to work in. I know a guy who still will only dive the Rat Hat to this day and refuses to dive anything else. He's an old North Sea hand.
As for the helmet, the condition is really good. I last dove it a few years ago. Everything works on it. It could likely take an inspection and polishing on the gel coat and chrome bits but it is solid. The EGS valve handle/knob was not installed on it when I got it. I had fit an extra Kirby Morgan knob I had lying around until I needed it for my Superlite. I found the original knob but just never got around to figuring out how it was mounted. The neoprene lining the shell interior is golden as is the neoprene on the neckdam. The neck clamp assembly is in great working order and the fiberglass has no defects. I also have a DM-5 shell and a bunch of spares.
The helmet has a Nostalgic history here in the Northwest. It was a helmet owned by Crowley Maritime back when they employed divers in the 70's and I believe into the 80's. As Crowley didn't want the liability and cost anymore the a handful of the divers decided on their own company and bought all of the equipment. Thus spawning the original owners of Global Diving and Salvage. Funny thing, I was just in their offices the other day and saw a picture with someone using an Aquadyne DMC-7 wearing a unisuit from well before my time. I believe it was one of the owners.