The 12AU7 is a double triode and this exhibit the 12AU7A is a special quality version of that design specification. Probably a rugged version for operation in a vibrating environment.
An amplification factor of 17 is quoted, the same as the 12AU7.
The Miniwatt brand and Made in Australia.
Through the dirty glass can be seen the twin triode sections. The double mica discs are obvious. These confer much greater resistance to vibration than does a single mica.
The thin glass tube envelope is 20 mm in diameter and the length excluding the B9A base pins, is 48 mm.
Reference: Data-sheet. Type 12AU7A was first introduced in 1956. See also1956 adverts.