The 6A7 is an American Pentagrid converter or heptode mixer. The 6A8 is the same valve electrically but on an IO base. We also have the 6A7E in the museum.
The lettering has partly rubbed off but the lower part of the final X matches our other 6A7 from Impex Radio.
The top mica showing the line of grid supports. The signal can clearly be seen to be introduced between the two grids that are strapped together - the screening grids.
The classic envelope is 38 mm in diameter and, excluding the UX7 base pins, is 98 mm tall.
References: Data-sheet & 1040. Type 6A7 was first introduced in 1933. See also1933 adverts.