Bizarrely, despite looking at them all my life (or maybe because of that), I'd not noticed that clocks using roman numerals often use IIII instead of IV. How can I not have seen that?
Also seems that no-one is quite sure why; symmetry of stroke weighting with the VIII, or having the first four digits using Is, the next four Vs, the last four Xs, or avoiding IV as it's an abbreviation for IVPITTER (Jupiter), or that it gives even numbers of each character (X, V, I) so if you are casting the numerals out of metal to attach to the face the mould is symmetrical.