I think this fits in here. I just picked up a 1898 Muaser converted to 28 guage. It also has a 2 round magazine coversion in the magazine well. The reciever and stock has been lightened.
The barrel is marked Gemo and barrel and reciever has the same serial number.
I assume these conversions were done between WW1 and WW2.
I any interest I can post pictures later. The only other shotgun conversion were in single shot 12 guage.