With iso 1600 film,exposing at iso 400 would result in a certain level of overexposure. This happened here,but mainly in the highlights,much less noticeable in the shadow areas - nothing major - certainly correctable in photoshop elements,and,from memory, in a wet darkroom.
Not to belabor the point, but did the highlights fall on the shoulder of the curve, so that added exposure did not produce greater density ?
Or did the highlights simply become more dense ?
This is worth considering: the nature of Neopan 1600 is to have a long shoulder
while it is the character of Rodinal to build a high density. I would expect that, given no agitation,
you might have reached a point on the curve where increased exposure would not build more density.
With a film like TriX, or Delta 3200, this wouldn't happen. Nor would it, I'm sure, if you had agitated at 5 or 10 minute intervals.
The term 'block' unfortunately has come to be used to describe the two opposite conditions: gross overexposure,
or having the tone fall to far up the shoulder . It would be interesting to which was the case, did the Neopan 1600 shoulder win out, or did Rodinal's classic highlight ? Or was it a draw ?