The face of the number board 'glass' part was "painted" using a black Staedtler Lumocolor Permanent marker, it provides the black surface yet gives it just enough translucency for the light to shine through as per the prototype.
The numerals are from a Microscale Decals sheet (number 87-1260 in this case):
View attachment 186098
Other sheets are available, UK availability is scarce however with my collection being made up of sheets found on ebay where modelers have cleared out their spares and some like this one direct from my preferred supplier, TMR in Canada.
Fortunately on the sheet where the number board numerals are, there was a 7396 at the end of four rows so I cut off the end 6 and added a 9 in its place.