The 4YSGAC from Six Sixty is also known as SS4YSGAC. The Mullard equivalent is the S4VB.
The Printing on the green metal loaded screening paint.
The early 1930s screw top cap with the screening box visible within. The clear glass S4VB shows the form of the internal structure.
The even at a greater angle the anode plate is still not visible - just the anode support that connects to the top cap. However, the main picture does show part of the anode under the peeling paint. Anode plates are placed either side of the screen grid.
The balloon envelope is 48 mm in diameter and, excluding the B5 base pins, is 126 mm tall.
References: 1931 data-sheet & 1043. Type 4YSGAC was first introduced in 1930.