3 3/4" tall...2" across...top hole 1 1/4"...bottom hole 3/4"...external threads...P 41 at start of threads. Front embossing: PYREX / T.M. REG / U.S. PAT. OFF. / MADE IN U.S.A. Rear Embossing: PAT. NO. / 2046960 / 7500 VOLTS.........................Thanks to Gene Hawkins and Douglas Moody. This is a High Voltage Electrode Housing patented in 1936 by Samuel C. Miller.