Dan, I don't know if you remember our thread on building a 1.720" bore Mallet's mortar or not, but it might help you visualize some of the parts and shapes of this beast of mortars. There is a conflict between the configuration of the breech presented in Starr 2011's diagrams and the photo we found of the actual mortar as it exists today at Fort Nelson, Portsmouth, UK. We are very appreciative that he posted that set. You don't see too many prototype drawings of historical ordnance. Perhaps the lack of conformity can be explained by a drawing, yet to be located. There are two photos showing a side view that illustrates a breech that protrudes below the heavy base plate. We located one of those which we posted below. Despite it's diminutive size, it clearly displays the structural elements of Mallet's Mortar of 1857. Only No. 2, at fort Nelson, exists today. Some "all rights reserved" photos in Flicker, et al show the same breech protrusion, but we can't use those legally, of course.
Good luck Dan. This is the link to our old "build thread":
http://www.gboreloaded.com/forums/index.php/topic,125169.0.htmlMike and Tracy
The little photo that reveals a lot. from
www.greenwich.gov.uk