The 12A7 is a combined audio output pentode and half wave rectifier. The pentode has an output of 550 mW and the rectifier has a rating of 30 mA. The valve is on an American UX7 base and the voltages clearly indicate that it was designed for use on American 110 Volt mains supplies. The target market seems to be local station radio receivers.
The pentode is on the left with the rectifier as a very thin assembly on the right. The heaters are connected in parallel.
Face view of the rectifier section. The base carries the RCA branding.
Thanks to Mark Mallison for sending the image.
The classic envelope is 44 mm in diameter, and excluding the UX7 base pins is 113 mm tall.
Reference: Data-sheet. Type 12A7 was first introduced in 1937.