The 5B/250A is an 807 with a ceramic base cap for added moisture resistance and a maximum anode voltage of 600 Volts compared to the 400 Volt anode maximum for the original 807.
The parameters given are for single ended use. When used in a class AB1 as a push pull pair with 600 volts on the anode an output power of 65 Watts was normal.
The 5B/250A Type designation is of the STC name style and Brimar is the consumer brand of STC.
The classic envelope has a maximum diameter of 51 mm and a length, excluding the UX5 base pins, to the tip of the top cap of 128 mm.
References: Data-sheet & 1040.