This exhibit is one of the Mullard 'PM' series valves, dating from c.1927. This is deduced from the fact that the base cap in the exhibit is not of the style used in the earlier PM series and the large 4 on the glass.
The large number 4 sits above the third type of BBC stamp.
The painted A and line impressed into the base cap is typically Mullard.
The anode is a square box, positioned horizontally.
Here the etched lettering is clearer. On the left is the Mullard log and on the right is the number 4.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins, is 92 mm tall.
Reference: Observation. Type PM4 was first introduced in 1925. See also1925 adverts.