The cited text has an extra detail: «… and it is customary to draw a line above all the letters making up the number, the dots of i and j being put above thi

The cited text has an extra detail: «… and it is customary to draw a line above all the letters making up the number, the dots of i and j being put above this line; for example, xviij» So, XVIII would have been written likes this: _____ XVIII ----- Lines would clearly delineate the beginning and the end of the number. This is how I have written the Roman numerals since school. So, if the script had X written that way, adding a C, would have also required both lines to be extended. Not impossible to forge, but an extra effort nevertheless.