The 12AT7 double triode is equivalent to the Mullard ECC81. This type of valve was designed for RF use and is of the low impedance type.
This valve is marked as Foreign Made. The envelope is the same diameter as the ECC83 made in the GDR and this may be the location of the manufacturer. Note also the bright support strut for the getter.
This is another Brimar 12AT7, but this time an original British made valve.
The British valve seen end on to the anodes. The getter support is an extension to one of the anode supports and will provide some extra heat sinking.
The thin glass tube envelope is 21.5 mm in diameter and excluding B9A base pins the length is 47 mm.
References: Data-sheet & 1040. Type 12AT7 was first introduced in 1947.