The U35 is an EHT rectifier. The anode voltage is given in terms of RMS voltage and this was probably used in mains based EHT generating equipment fed from an EHT mains transformer. The voltage is low for television use but ideal for instrumentation such as oscilloscopes. The reservoir capacitor specified is 0.001 µF.
The glass U support is seen above. This gives additional insulation.
The U35 image on the envelope.
The wide glass tube envelope is 28 mm in diameter and, excluding the IO base pins, is 89 mm tall.