After seeing the photo that Bill Ostrander posted of the porcelain ring insulator last week I had to follow up with my own photo. This device is a service drop insulator, we called them "porcelain donuts" back in the 1980's when there was a bunch of them in use around the San Antonio Texas area. It was common practice in the 50's and 60's to use a copper insulated cable as service drops to many houses around San Antonio, this cable was #6 solid soft drawn with a heavy cloth / varnish insulation, and this cable was run through the donut and wrapped back on the cable with about 6 wraps to dead end the cable to the donut. It was common to have 3 to 4 cables per donut and 3 donuts per pole, 2 hot (120volts) and 1 neutral. This would supply either 120 volt or 220 volt service to the house. We removed tons of these back in the late 80's to a more modern rigid clevis and 3-inch porcelain spools. I did keep one of these for my own collection, should have kept more of them! The flat piece of strap went between the rigid clevis and the 3-inch spool, the clevis pin that held the 3-inch spool also held the service donut in place. Check out the following pictures.