The 6AM5 is a miniature audio output pentode and in this Mullard exhibit the American designation shares the front of the envelope with the EL91 Philips/Mullard code.
In single ended configuration the 6AM5 / EL91 will produce 1.4 Watts, enough for a small broadcast receiver.
The anode is carbonised to radiate heat more efficiently. Output valves in the 18 mm tube of the B7G format ran very hot.
The grid supports above the top mica show the relative spacings employed.
The thin glass tube envelope is 18 mm in diameter and excluding the B7G base pins, is 47 mm tall.
References: Data-sheet & 1040. Type 6AM5 was first introduced in 1949. See also1949 adverts.