The UR3C is electrically identical to the UR3 that has a Ct8 base.
At first sight a full-wave rectifier with a series-type heater apparently for AC/DC use seems to be self-contradictory. However, this is one of a small number of types which were intended for use in voltage doubler rectifier circuits (AC only!) in which around 200 VDC could be obtained from 110 VAC mains without using a mains transformer. Alternatively the pair of rectifiers can be run in parallel for greater current output.
Another curious feature is that it has a Philips/European 'U' series type number (where 'U' indicates a 100 mA series-type heater) but its heater current is actually 200 mA!
In fact the Philips 'UR' series were ephemeral except for Type UR1C (with B5 base cap) which in redesigned form remained in the lists until the 1960s. The UR1 (with side contact base cap) was also redesigned but was renamed the CY1 in accordance with its heater current rating. The other 'URs' were little used and were obsolete by 1940.
The twin anodes.
The working part of each anode is towards the centre with the outer parts being heat dissipating flanges moulded around the twin supports.
The classic envelope is 40 mm in diameter and, excluding the Ct8 base pins, is 100 mm tall.
References: Private communication, datasheet & 1043. Type UR3C was first introduced in 1935.