Unlisted embossing. The embossing is extremely faint and the whittled glass makes it even more difficult to read the embossing. Using digital image enhancement I was able to make out WCC 10 and about one inch to the right (in an arc around a half circle shape) CAEAAHO B CCCP / 1989 (or 1969?)

The half circle shape is not the typical embossed insulator that is commonly found on Russian insulators. It looks like a "D" rotated 90 degrees counterclockwise with another "D" in the middle.

[IFONA CD 247.5]