As far as I can remember from Essery and Jenkinson LMS Locomotives vol.5 the lettering was hand-painted straw - I'm a few miles away from the book right now but I can check properly tonight, it might even have the letter and number sizes. They should be on one of the HMRS sheets.
I think I confused it with another loco I was looking up - E & J suggest gold with black shading 'C13' style which is 10" numerals. I think that's HMRS sheet 1