From Popular Mechanics Magazine - July 1919

Note the description. They are used just like battery rests where they fill the area between the skirt and the pin with liquid.

Text reads:

Glass Insulators Keep Insects from Refrigerator

An efficient method of keeping ants, and other insects, out of the refrigerator is to mount it on legs made of common telephone line insulators, screwed on the brackets made to be used with them. The hollow part of the glass insulator is then filled with some liquid, like kerosene or turpentine, which the insects will not cross. Another advantage of such legs is that they are adjustable in length, so that the height of any one of them can be changed slightly to make the ice box stand steady on the floor. -J. G. Zimmerman, Chicago.

[id: 651680300; Original] found by Eric Halkyard; this is a larger example.