Apparently clocks used to say IIII instead of IV. How common was this? When did they start commonly using IV instead of IIII? Where there any other numerals on clocks that used to be written differently?

They still do. Source (there is a mix between iiii and iv notation): https://www.amazon.co.uk/Roman-Numeral-Wall-Clocks/s?k=Roman+Numeral+Wall+Clocks So clearly both are commonly bought and acceptable.

Both ideas are easy to understand, so can be used interchangably. However the '4' (IV) position is tilted slightly upside down, and the '6' (VI) is completely upside down.

When viewed on a wristwatch or sundial the numbers could also be viewed at an angle. So the IIII removes confusion between trying to work out the difference between an upside down IV vs. VI.

Older post: https://www.reddit.com/r/AskHistorians/comments/8v75yk/why_do_clocks_with_roman_numerals_have_iiii/ by u/Weemstar Old reply by u/qed1 : https://www.reddit.com/r/AskHistorians/comments/8v75yk/why_do_clocks_with_roman_numerals_have_iiii/e1lj6io/