Unlike a normal Waterman spoon feed, this nib has two side channels flanking the central channel, the side channels vented both out the front end and through the feed's bottom by means of two slits.
Another apparently unique feed design related to this one was found on an 8-size Waterman that was clearly being used to test all sorts of experimental features (belatedly described here). That feed is shown above. As I noted in a comment added to the Facebook post:
"The other, 8-sized example was actually quite different, in that it was proportioned more like the early narrow feeds, widened just enough to fit in the two side channels. The side channels were also vented below via slots, but did not extend out the front as here. The feed tail was similar. Note that vents extending from the side channels to the underside of the feed are a standard feature of many production Waterman feeds for their larger pens (8 and 10-size), but in the form of a pair of small round holes." [pictures now added below -- D.]Then Olle Hjort wrote to suggest I take a look at Weidlich's US patent 760,829 of 1904. I think he's on target, and that the two slit-vent feeds are connected to Weidlich's design in some way which remains to be clarified. The Waterman versions do differ in design from the feeds shown in the patent diagrams, but clearly incorporate their key features. Could Waterman have been evaluating the design with an eye to licensing or buying the patent?