The 1X2B is an EHT rectifier for television service. The anode voltage quoted is the peak inverse voltage and this is important in a television EHT generator as the waveform is pulsed and not a sine wave.
The R12 has a higher specification than the 1X2B and can be used as a direct substitute for the 1X2B.
The Raytheon logo and Made in USA.
The deep bell anode connected to the top cap and supported centrally by two mica discs.
The lower screening disc is close to the anode and the cathode is not visible. The central mica disc is clearly seen in this image.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 62 mm tall.
References: Data-sheet & 1040. Type 1X2B was first introduced in 1955.