Tuomo Pekkanen's Latin grammar mentions (§52.3) that the -e- added to the present stem before -nd- in the gerund and gerundive (in the third and fourth conjugations) can be replaced with a -u-. For example, mittendus = mittundus and faciendus = faciundus. No more details are given.
When can I one replace the typical -e- with -u-? Is this specific to some era, author, or verbs? Would it be appropriate to occasionally make the replacement in any context?
I wanted to understand the -e-/-u- variations in gerund(ive)s better. The variations can also be across words, not only within single words; I consider both variations to be essentially the same phenomenon.