And as per the instruction of 1919 "it is customary to draw a line above all letters making up the number, the dots of i and j being put above the line" (in other words, the horizontal line is directly above v, x, c, and so on but between the dot of i and j and the lower part of the letter).

I think this way it is difficult to add more Roman digits like x to fake a higher amount than prescribed.

The letter j seems to be an additional security because i as a letter is narrow and one could perhaps squash in one or two more i's. The instruction also mentions that the number of dots is important, if the number of i and j is not the same as the number of dots above the line, the apothecary should assume an error.