Steve, I use Guardian service as my normal day-to-day cookware. It is heavy (about 1/4" thick), heats very evenly and lets you cook at a low temperature. It seasons much like cast iron but doesn't have a dark build up.
Your link to the site is a good one, the information is pretty complete although not fully accurate, but you would have to be a fairly picky specialist to catch it. In example, Century Metal Craft, the Guardian Service makers (and Silver Seal before 1936) started in Chicago. The fire in LA is what finally stopped them.
There are at least 4 trademarks that I have had and have pictures of, not just 3, but this is really just for a specialist to worry about.
Regarding cleaning, I've done pretty well with aluminum (not just Guardian Service) doing the following.
1. Remove any non-aluminum hardware such as handles and knobs.
2. Run through my stove's oven cleaning cycle. (this normally gets everything off, but there will still be some dark spots on it (tarnish).
3. Boil in a Cream of Tartar bath for 30 minutes to remove the tarnish (2 tablespoons to the gallon). (Don't go much over 30 minutes, this is a weak acid and will even pit the surface if left way too long.)
4. Buff the piece with 0000 steel wool, wet or dry. This is really the finishing touch, with hammered finishes it does a spectacular job. Does great on the polished surfaces too. I've tried jewler's rouge, but the 0000 steel wool works best for me.
I won't use power tools of any kind when working with my aluminium. They seem to give an unnatural polish to the piece. It's hard to describe and probalby needs to be seen to understand.
For those pieces you can't get the handles or knows off of, I will try various oven cleaners that say they are safe for aluminum, or some paint removers. Or as I've recently done, use walnut shells in my sandblaster. This is magic the way it cleans and doesn't damage.
For most people, you should stay away from lye or ammonia based cleaners. It cuts grease nicely, but ammonia will turn the item black and lye will eat it up. You CAN use these products but you have to measure the time in minutes and watch it closely.
I have an example of a lye bath experiment that I forgot. I have a pair of alaska handles that were riveted to the aluminum waffle iron and a cast iron high base. The aluminum became scum at the bottom of the barrel because I forgot it for two days.
You need to take a cheap piece and experiment. I've had uneven staining and discoloration on an aluminum pot that I've popped into an ammonia solution briefly to clean and turn an even black. Then into cream of tartar to turn back to white aluminum, then steel wool. The result is an even, new looking finish.
I wish I had thought to take before pictures. Yesterday I cleaned up several pot/skillet lids (guardian, silver seal, and magnalite) in this way. Most look brand new, all that is left on them to show age is the normal nicks and bumps from being around so many years.
Tom
By the way, this works to clean badly burned pan contents. I tease my wife that she only knows two speeds on the stove, off and high, and uses the smoke detector to let her know things are done. The oven and steel wool save a lot of elbow grease, even with a user.
