I found this insulator and bracket on sale at an antique store in Broadway in the Cotswolds. The price was £9, or about $13.50. At least I was able to get a specimen legally!
Later: I was able to get this one off the very rusty pin. I was concerned about putting a lot of torque on it, as about half the pin was rusted away. You can see how much of the bracket is gone in this picture:
Since rust has about ten times the volume of iron, the insulator was locked on tight. First, I reduced the iron oxide with full-strength muriatic acid. I put the insulator pinhole side up in a covered plastic bucket, and filled the cavity with acid, thinking that the acid would work down around the threads.
After a day, I rinsed it thoroughly and put it into the oven, slowly raised the temperature to 350 degrees, put on my welding gloves, took it out of the oven, sprayed a little WD-40 onto the pin, and it screwed right off.
I don't know if 350 is hot enough to soften any goo that might have been used on the pin threads. I didn't have to deal with that. But with a good pair of welding gloves, temperatures higher than 350 should not be a problem. The coefficients of expansion for iron and porcelain are close; iron is about 0.000006 and according to the Web, porcelains vary around 0.0000053 and 0.0000067, depending on formulation, so somewhat higher temps should not crack the insulator when the pin expands.