I've just installed a 1555 nib on my SJ and tried it out. Writing normally with it, it seems like a fairly typical medium nib - nothing out of the ordinary other than a slightly greater scratchiness.
If it feels medium, it may just be worn. The 1555 is an untipped nib, and a lot of these old Estie nibs are quite broken in and probably don't write anything like they did when they were new.
BTW -- Does anyone know if any of the Esterbrook design prints are available? The patents should be expired. Perhaps some entrepreneur could start remaking "Renew Nibs" again -- or under license from Berol, -- that I presume own the patents – and have the drawings.
As with many of the old companies, I think all the records were destroyed when the company was sold to a non-FP company.