Arkansas Pen Club member EuGene Smith recently came across this fascinating Boston Safety Fountain Pen 53s and shared it with us at our September meeting. That protruding metal piece is why they're called "hump fillers." The hump on this one appeared to slide back and forth to unlatch the mechanism and allow it to be depressed, compressing the sac inside.
The mechanism on this one was a little stiff, and as EuGene had just purchased it, we decided not to push our luck and didn't try to force it. We found a post on a web forum that said Boston used two digit codes on their pens, the first digit being the type of filling mechanism, in this case the '5', and the second digit being the size of the pen/nib combination, in this case a 3. The 'S' at the end referred to the fact that this pen came with a slightly smaller nib than typical. So, this 53s would mean a 5-type hump filler, a 3-size pen, with the s indicating the nib was a 2 rather than the normal 3.
As you can see in this close-up, the Boston Safety Fountain Pen imprint actually runs beneath the metal band of the filler. EuGene speculated that Boston took ready made barrels destined for other pens, which had already been stamped with the imprint, and modified them for this line. Makes sense to us!
This close-up shows the four leaf clover pen clip, which we are almost certain is not original. We have seen this same add-on clip on other brands of pens, particularly a Manos we came across recently.
Thanks again, EuGene, for sharing this wonderful pen!