The 25Y5 is a pair of half wave rectifiers. The design has a greater spacing around the cathode compared with the 25Z5 to allow for European mains voltages and thus the 25Y5 was aimed at export rather than the US domestic market.
With a 300 mA heater requirement universal sets without a mains transformer would be the target market. In sets without mains transformers a full wave rectifier was not appropriate, however, it could be used as a voltage doubler on 110 Volt mains or as a paralleled pair on 230 Volts.
The base cap carries the Arcturus brand. The style of octagon etched into the glass is identical to the RCA design.
A closer view showing the anode/cathode spacing. This can be contrasted with the 25Z5.
The classic envelope is 38 mm in diameter and, excluding the UX6 base pins, is 88 mm tall.
References: Data-sheet. Type 25Y5 was first introduced in 1941. See also1941 adverts.