Vintage 3" Ceramic Insulator or Standoff for High Voltages

Description

Vintage 3" Ceramic Insulator for High Voltages. Includes plated hardware shown.
Note that the hole in the bottom only extends about 1/4". It does not go the length of the insulator. It is not clear to me how this part is mounted or what the purpose of the rubber washer might be.
Condition is very good, possibly even NOS, but I am calling these 'used' to avoid any disappointment..
Shipped with USPS First Class.