Interesting. I have run across mention of these weapons in a couple of articles/books in the past, but have never seen any specific details before. The only details I ran across were something like 'fired from tube launchers' and 'operated similar to the British aerial mine'.
The first time I read about the AA mortar was in an article where it was believed that an A-26 had its wing blown off during a strafing run. Multiple small parachutes were seen to appear just before the explosion.
The weapon was taken seriously enough that it earned mention in the March 1944 Air Force magazine.
The British had the Holman Projector (naval and land types), as well as the multiple barrel 7-inch UP mounts on larger ships and ashore in a few harbour installations.
The Japanese also tried dropping WP bombs over our heavy bombers but it did not seem to be very effective. When George II fighters attacked the two PB4Y-2's of VPB-109, they began by dropping the bombs, to no effect.