just got one of these...brought it home and ended up with a broken back
do i absolutely need the drop-in filter for this to work properly???
I'll assume you have the filter holder, otherwise you'd have a light problem, via the rectangular hole in the barrel
I have read for sure that the filter is part of the optical formula in the reflex-Nikkors (500mm and 1000mm), don't know if it's the case with the 400mm, but to stay on the safe side, you better have a filter in place...
The drop-in filter is 39mm thread, and usually you can find it in the used market, especially colored ones (orange, red, YG, ...)
If you find a colored one, you can replace the glass with one from a standard 39mm clear filter. Easy-peasy, just remove a thin steel wire that's holding the glass in place, replace glass, put back the steel wire and you're done.
The thread is special, resulting in such a way that the filter is screwed entirely in the filter holder, with the edge of the thread flat against the holder border, and there is no protrusion.
You can try fitting a thin standard 39mm filter, if it doesn't stick out too much you are good to go.
I have recently bought such a lens myself, good in every aspect except the crummy short hood. I'm building a proper two-segment hood, more details in the
What the Nerds do section when I'm finished. Filter size is 122 mm (in my sample, a clear lens protector was included: seems the front lens is soft ED glass), I had to use a larger tube (155 mm) to avoid vignetting, with a scaled cylindrical adapter going from 125 to 155 mm.
Ciao from Massimo