Disc size
It's 9" in diameter and the holes are 6" apart (not 5").
What year is your truck? These discs were pretty common on 11 and 12 cabs but only the earliest 13 cabs seemed to have them. By '43 onwards, the yellow bridge class was painted directly on the right headlamp cover or, even later, the flat space between the right headlamp socket and the grille.
Cover and flat space:
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