Cybermat47's WIP mod brings back and old question:
when the first Allied merchants started being equipped with guns?
I remember having read a BdU standing order suggesting caution with deckgun's usage even against unescorted merchants. According to it, the deckgun should have been used only as an emergency solution, or for administering the classical "coup de grāce" against obstinate merchant targets. If memory serves me well, the reason was avoiding being identified by the enemy, and avoiding unnecessary risks.
This would have involved that by the time the aforementioned order was issued (I don't remember exactly the date, but it had to be around early-mid '40), the first armed merchants had started to appear.
Anyway, whatever is the answer to the above question, the following links might contain some good clues:
http://www.uboatarchive.net/BDUKTB.htm
http://www.uboatarchive.net/KTBList.htm