Here's some pic of ww1 trench mortars.
Pic 1 - 3" Trench mortar with fins missing
Pic 2 - 4" Trench mortars with fins
Pic 3 - 6" Trench mortar with fins
Pic 4 - Unusual one here - it's a 4.5" shell which has been converted to a 6" mortar, with the addition of 2 rings to increase the diameter to 6" and act as gas check rings and a tail welded to the base of the projectile. You can see the filling hole in the side of the projo.