The Austro-Hungarian empire was heavily involved with such mortars, they are detailed on this fine (German language) page.
Having had the opportunity to look at surviving examples of the 12 cm Luftminenwerfer M.16 and 20 cm Luftminenwerfer System Bartlmus preserved at the Heeresgeschichtliches Museum close up, I thought I would share some photos. Not my best effort, flash photography was not allowed.
Unfortunately I was unable to get any details of the clutch, but the function can be easily imagined as a quick-release mechanism which releases the projectile when pulled.
One thing I don't understand is how the projectiles are stabilised. Having seen the studs on the projectiles, my initial idea was that they engaged rifling in the barrel in a manner similar to the La Hitte system, but in actual fact the barrels are smoothbores, and the studs do not project beyond the major diameter of the projectiles.
Looking at a cross section, the CG and CP are about central, so it doesn't appear that it would be drag stabilised.
The most plausible answer to me is that the place where the projectile sits actually contains a short rifled section. Even thought it's only over half the length of the projectile, this could provide enough spin to stabilise the relatively short projectile in flight. The 20cm model had its breech open but a projectile was in place and I was unable to confirm this. Any other theories welcome
One other interesting thing is that the 12cm model has a spring loaded shutter at the muzzle. The springs look fairly weak so I imagine this was mostly to keep debris out when not in use, but it could also have had a limited suppressing effect.
I digress, herewith the pictures:
"yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " 
what?
That big fat metal projectile, with a relatively short barrel goes 1km...
