afaik there are minimum of three types of feeds in Esterbrook nib units and the one you mentioned i guess is maybe a malfuntioned feed as i also haven't seen one like yours. Here is a image showing three different feeds http://www.fountainp...nibs/?p=1822453. you could also fit in a #5 feed from other pens into the estie collar but afaik there is no other company that made estie feed lookalikes except venus.
Not a malfunction. I found many these feeda naturally occuring on the "transitional" (flat barrel end, J-pen cap) pens I picked up, and so, based on their design and natural occurrence, place them historically between the "fat-flat" and the rounded, "Sheaffer-style" comb feeds of the later points; (late- or early post- WW2).
For some goofy reason, the feed/nib reminded me of a WW2 PT boat, so my warped logic dubbed them "gunboat" feeds. My shrink is still puzzled over that thought process.