Based on the Marshall and Oldfield repair book, when you push the button, the sac in the back end twists, filling the front half of the barrel as it untwists. So it is a push button twist bulb filler. Repeated pushes are needed to slowly fill the barrel. There is no spring and they state that you have to push and pull for each stroke. Other twist fillers return due to the elasticity of the twisted sac, but I can see how this design might have too much friction to allow for that to happen.
I do not currently plan to restore this pen. It took me years to find one of these pens, and the one I finally got is just so clean and perfect that I hate to mess with it. If I ever restore this one or find one with flaws to restore, I will share what I find with all of you.
According to the Moak book, there is no patent. It seems to be a very uncommon pen. So much so that the pen is the Marshall and Oldfield book is the same as the pen that appears in the Moak book.