The subtractive rule in Roman numerals and older variants of their notation

At times one comes across original formats of Roman numerals that differ from the familiar ones.

/For the sake of clarity, in the explanation below I shall separate the orders of magnitude with full stops; this is permissible/

As regards the notation we are accustomed to, 1899 ought to be written M.DCCC.XC.IX, and not M.DCCC.LXXXX.IX. As you can see, the difference lies only in the writing of the number 90. If we proceed from the way Roman numerals are customarily written in our own day, then yes, it would indeed be handier to write 90 as XC. Here the so-called subtractive rule applies. From 100 (C) we subtract 10 (X), which is shown in the notation by placing the number to be subtracted to the left of the one it is taken from. The rule is reinforced by a second norm, whereby in Roman notation one and the same numeral must not be repeated more than three times.

These rules, however, were not always canonical. For example, right up to the nineteenth century the number 4 was quite often written as IIII. In our day this form survives on some clock faces, as a charming archaism, perhaps connected with a wish to avoid invoking the name of the god Jupiter (IVPITER), whose spelling begins with IV.

I do not know whether this can be counted an error, but in some old documents one does in fact come across other ways of writing Roman numbers. For example, 9 is VIIII, 80 is XXC, 99 is LXXXX.VIIII, and 999 is DCCCC.XC.IX