The 12BA7 is a heptode mixer known in the USA as a Pentagrid converter. The attached data-sheet dates from 1948 and gives initial characteristics. With a 6.3 Volt heater the valve is the 6BA7.
As a reseller, Zaerix did not manufacture this exhibit.
The gaps in the anode wall reveal the wire grids within. The outer suppressor grid can be seen to have a wide pitch.
The lower gap in the anode shows several of the grids.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 60 mm tall.
Reference: Data-sheet. Type 12BA7 was first introduced in 1948. See also1948 adverts.