Fred Locke's Attic (page 3)
This appears to be a standard Imperial U-923 until you turn it over. The large opening is for a glass or porcelain base to be cemented in with sulfur. Note the "teat" on each of the four crown points. These were the locations of the holes in the crown part of the mold to release air when the insulator was pressed.
This is how the crosstop was put together. Fred Locke used a standard Brookfield pony ground down to fit. One specimen had a Brookfield pony that was not ground down. A new U-number will be assigned.
This new style will receive a U-number. Note it is designed for a glass or porcelain base cemented in the large opening with sulfur.
The new style is similar to the one above. A new U-number will be assigned.
The large insulator above is another new style that will receive a new U-number. It was also designed to have a glass or porcelain base cemented in with sulfur.
I didn't get a good photo of the two types of porcelain bases found so far, but you can see them in the photo above with the Brookfield pony that was ground down to fit in the porcelain crosstop.