The American Type designation of 12AT7 is equivalent to the European ECC81. This type of double triode was designed for RF use and is of the low impedance type. In the 21st century the 12AT7/ECC81 is much used for audio equipment. It has low noise and a gain between the ECC82 and ECC83.
The twin triodes with a central arm supporting the getter holder.
The folded anodes are stitched on one side. Tabs pass through slots in the mica discs to hold the anode in place. The lower tab has the connection welded to it.
The thin glass tube envelope is 20 mm in diameter and excluding B9A base pins the length is 46 mm.
References: Data-sheet & 1040. Type 12AT7 was first introduced in 1947.