The MX40 is an 1934 heptode frequency changer. This example has the grey metallic zinc paint over a gold under paint - odd.
The zinc paint carries the Marconi branding.
Looking closely at the space above the base one can see a wire encircling the glass that acts as the connection to the screening paint. Also there is some black lettering on the gold paint.
This is another valve with gold paint and the same wire style of connection to the paint. This image is from the Mullard FC4 exhibit and appears to show that M-OV were buying Mullard valves and re-spraying them.
The oscillator voltage was designed to be 10.0 V peak to peak.
The balloon envelope is 45 mm in diameter and, excluding the B5 base pins, is 117 mm tall.
References: Datasheet, 1040 & 1043. Type MX40 was first introduced in 1934.