The Pen3520 from Mazda is an audio output pentode designed for use in local station AC/DC broadcast receivers. With a high mutual conductance the Pen3520 could be fed directly from the the diode detector and thus save an intermediate audio voltage amplifier stage.
It should be noted that the Pen383 that is given as sensibly equivalent in the literature, is on an MO base and not the B7 of the Pen3520 and additionally has differences in rating.
The identification is etched into the dome of the envelope.
The bar at the top holds end of a ribbon that supports the top of the two hairpin heaters. The heater wire is in fact made from a close wound coil of thinner wire with ceramic insulation. The cathode is a rectangular box
The classic envelope is 54 mm in diameter, and excluding the B7 base pins is 115 mm tall.
References: Datasheet, 1040 & 1043. Type Pen3520 was first introduced in 1934.