I remembered a drawing in my collection but no details. I can however give you the dimensions of where the markings were placed on the Yp from a copy of Mbr 21818.
The start (right edge) of the lettering shall be 200 mm from the doors opening edge.
The bottom edge of the lettering shall be 60 mm above the bottom side edge.
The width of the lettering in all shall be 275 mm.
The heigth shall be 150 mm.
Behind the square formed above there shall be a letter "B" (acc. to Mbr 25849) that is 25 mm to the left of the text box, i.e. 200+275+25 = 500 mm from the door opening. The letter shall be 100 mm high.
Lettering in green directly on the paint, and for GJ (as in the picture) the lettering shall be in white...
Letters according to Mbr 20910
The road number acc. to Mbr 25849, 100 mm high (this is for the lettering on the side, below the SJ marking)
Not what you asked for, but maybe it helps...
With a little luck you might be able to find the correct Mbr drawings via the Järnvägsmuseum, if you need (or just like) help please E-mail me (anders.habo(at)gmail.com) and I can try in Swedish...

I dont have the referred drawings in my collection, maybe someone else?
Regards,
Anders Östlund