Thanks for the link! I was not aware that the museum's pages. They seem to have a lot of interesting stuff on them
U-boat commander's handbook has a few sections where it restricts the usage of deck gun.
BdU's war diary in late October 1939 has something on this too:
27.10
In order to lessen our losses and based on the reflections set out in the War Log on 23.10, I have decided to issue the following orders:
1) Boats may not board ships which have been stopped, for examination.
2) Ships are to be sunk by torpedo only, even when they are only attacked after examination (which can now only consist in looking through the papers) or after resistance has been quelled with gunfire.
I don't know what the wording of the order has been, but I suppose it has given the commander's some room to consider how, when and where they will use other armament than torpedoes.
The attack on Robert L. Holt might have been made without knowing how many guns the ship actually had but it seems that I might consider making some deck gun attacks against armed merchants from now on
