Model 53S, Boston Safety pen, from the era when a screw cap meant “safety”
US patent 772554 James Bullard Oct 18, 1904 (filling system)
The pen also has pat Sep 2, 1913 imprinted (This seems to apply to a patent 1,072,073 by George Brandt of Boston for a stud filler like the Jaxon/Conklinette with a slightly different way to lock the stud)
This is an uncommon pen with a simple lock. The hump has a deep undercut notch, and a metal sleeve outside the barrel can be slid under the hump to lock it, and out of the way to allow the hump filler to operate. This pen has the most industrial look of the hump fillers presented here, with a metal hump and large metal external lock. It is quite secure and easy to operate, but in my opinion, lacks grace and charm.