I have a little bit to add to this thread now I've drawn on the expertise of Henry Simpole.
The pen I bought cane equipped with a dip pen nib (I didn't think to look at first), which was quite dysfunctional with this lever-filler. Henry supplied a working nib, and all is now well. If this happens to be the same pen as described earlier in the thread, though, I would simply add that someone was misrepresenting the pen; it is neither an eye-dropper filler (very obvious lever) nor suitable for a dip pen nib.
I'd be very thankful, though, if anyone could add to what little has been turned up so far about the manufacturers of The Sheriff.