From my experience and the info i have, there were 4 mks of 9.45" mortar.
They were as follows
Mk 1 - Short body (Picture 1).
Mk 2 - Large body with cut off fins (Picture 2, top mortar).
Mk 3 - Large body, longer fins and smaller internal fill space (Picture 2, bottom mortar).
Mk 4 - Intermediate size body (Picture 4) - I'm pretty sure this one was made from cast steel like the 6" trench mortar (But will happily be corrected if wrong). You can clearly see the gas check rings on this one.