I was recently asked to send some photos to a friend, and as long as I had them out I giured I'd go ahead and post them, with a little of the history behind them.
The US went through significant mortar development during WWII, today everyone remains very familiar with the 60mm, the 81mm and the 4.2-inch. What has been largely forgotten, however, is the US 105mm, 155mm, 250mm and 914mm mortars.
As the US began fighting the Japanese in the Pacific, we rapidly discovered that much heavier mortars were needed to deal with the caves and fortifications being used by the enemy forces. The result was the new series of heavy mortars. As far as I have been able to find, all but the 914mm were deployed in the Pacific, the 914mm was reportedly ready to go for the invasion of the Japanese mainland. There are also indications that the 155mm at least was deployed to Europe, I saw one of the mortars in an EOD training collection - I believe it was at Shrivingham - in around 1998.
I have three examples of these projectiles in my collection, a 105mm and two different models of 155mm. Nearly all of the series take the M4 rocket fuze, which did not require spin to arm, but one of my 155mm takes a smaller fuze, which I suspect would have been the M77 or something similar