Pretty blue striper. It has a circle incuse on the dome with a line through it and either 66 below it, or 99 above it, depending on how you look at it.

Information from Zoltan Drinoczi:

That is a Hungarian Striper from the 1950-60's of the railway telecomminication circuits. Manufacturer: Hollohazi Porcelangyar (Porcelain Factory of Hollohaza, H in circle with two numbers, the 66 is the manufacturing year)

Near the railways used to run the telephone lines in ownership of the mail. The railway used to sign their own circuits on the foreign lines.