The RR3-1250A is a high voltage xenon half wave rectifier for industrial applications. The PIV is 13 kV and the peak cathode current is 5 Amps but 1.25 Amp continuous.
Mullard say: 'Xenon rectifiers can be operated over a wide ambient temperature range, they are not restricted to vertical mounting, they have a short heating-up time and require no 'conditioning' on first being put into service.' Sounds a better bet than a mercury vapour rectifier.
The Goliath Ediswan Screw base can be clearly seen on the GXU4.
Thanks to Andrew Birkett for the photograph.
The wide glass tube envelope is 60 mm in diameter, and including the GES base is 260 mm tall.
References: Data-sheet & CV2399. Type RR3-1250A was first introduced in 1957. See also 1957 adverts.