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 top mica shows the five grid supports and that the visible grids are wire. The grid two support has no notches present and this is because it is just two plain rods that act as the oscillator anode.
A closer view of the top of the valve, grids and coated cathode tube. The cathode tube seam is visible at the top and inside the grids the seam appears as a dark line.
The classic envelope is 38 mm in diameter and, excluding the UX7 base pins, is 97 mm tall.
References: Datasheet, 1040 & 1043. Type 6A7 was first introduced in 1933.