Mullard was completely taken over by Philips in 1928(!) so any documents might be in the Philips Company Archive in Eindhoven. Their email address is:
pca@philips.com
but the problem is you have no designation for the valve. I suggest you scrutinise the valve's bulb with a magnifying glass, possibly with a light behind it which might show up the permanent ink marking of the code (if there is one).