The 12AU7 is a double triode with separate cathodes. An amplification factor of 17 is quoted. The 6C4 is half of the 12AU7. This exhibit has the white Brimar logo and 12AU7 has orange lettering.
The two triode sections.
Made in England and Brimar were part of the BVA (British Radio Valve Manufacturers Association.)
The thin glass tube envelope is 20 mm in diameter and the length excluding the B9A base pins, is 48 mm.
References: Data-sheet & 1040. Type 12AU7 was first introduced in 1951. See also1951 adverts.