The 12AU7 is a double triode with separate cathodes this example carries the alternative Type designation of ECC82. An amplification factor of 17 is quoted. The 6C4 is electrically one half of the 12AU7.
The twin triode sections.
One triode face view, almost half of the width is taken by the flanges that both stitch the two side plates together and help dissipate the heat generated in use.
The thin glass tube envelope is 20 mm in diameter and the length excluding the B9A base pins, is 48 mm.
References: Datasheet & 1040. Type 12AU7 was first introduced in 1951.