Mine has been altered to suit a change in the premises - and definitely not by a 'thousand-job apprentice'!
According to my January 1929 SUNCO catalogue, annunciator screens were provided in glass or zinc, with glass being black and gold. Writing the names of the rooms cost 6d per signal, and writing the fixer's name and address on the glass cost 1/- extra.
Here's mine. Note the 'amended' writing.