The 12AU7 is a double triode with separate cathodes. An amplification factor of 17 is quoted. The 6C4 is half of the 12AU7.
One of the anodes showing the stitch that holds the two halves together.
The thin glass tube envelope is 20 mm in diameter and the length excluding the B9A base pins, is 47 mm.
References: Data-sheet, 3002 & 1040. Type 12AU7 was first introduced in 1951. See also 1951 adverts.