I have not handled one of these pens. There is an article in The Pennant late 2024 about these pens. There is also a website with pictures and description here.
The pen has a button on the side mounted in the middle of a decorative plate. There is a small slide under the plate that releases the button so the sac can be compressed. Pictures credit to munson pens (munsonpens.wordpress.com)
There are three applicable patents. The second closely resembles the production model.
US 896,576 This patent is very much like the Boston hump filler sliding lock
US 955,475 The production model with the button in a frame and a hidden slide release
US 1,036,149 in which the button does not lock, but is shielded by a surrounding short tube. The cap top fits this mechanism and is used to press the side button inside the short tube. This would be like a hybrid matchstick filler with the matchstick recessed in the cap, combined with a side button.
