The 12BA6 is a variable μ pentode for use at HF within AC/DC receivers. Most probably as RF amplifiers. The 300 mA heater is clearly rated for series heater chain use.
The RCA brand lettering.
The anode obscures most of the inner details, here is a glimpse inside through the hole in the anode side wall.
The thin glass tube envelope is 18 mm in diameter and excluding the B7G base pins, is 46 mm long.
References: Data-sheet & 1040. Type 12BA6 was first introduced in 1946. See also1946 adverts.