I suppose we would need to resurrect the ghost of some long departed Danish ordnance officer to learn the reason for this. Any military ammo, incl 173grain match loads which are 3.330 OAL, will load into the magazine just fine. Millions of rds of the longer 172gr ball ammo were fired in M1917s during WW1 without a problem. A number of the Danish M1917s were from Cdn WW2 surplus and some have speculated that the Cdn Army might have done this. I've never seen it on a Cdn surplus M1917, but I have seen it on a Cdn surplus M1917 that was used by the Danes.
Seems like a solution to a non-existant problem. I've never tried to open a bottle of Tuborg beer in the notch, but seeing how the M1917s were used by Danish reservists, maybe it was deemed to be an operational necessity at the end of a long training day.