Alum. & Steel [also made in Brass] screw bases are most likely for Crowns [6" base to 7"]. I'd bet he is looking for a Threaded HULL 5" to 6" Copper/Brass screw base.
Like DB posted, screw bases for Crowns have a 6" threaded opening with the bottom flange being 7" across. On standard glass bodies the threaded opening is 5" across and the bottom flange is 6" across.
Hi, champlinman (and others). The copper screw base has been reproduced...at least in the 6" bottom flange size, which fits a regular glass body. I believe they are about $45-$50. Cheese, Gas Pump Heaven, Charlie Pergl, etc should have them. If the 7" size is what you need, I would at least ask these guys who sell the 6" size, if they can help.