Your 10 minutes would be about right for a *fresh* NP20 in Rodinal 1+40. However, as you cannot assess what has happened to the film and, above all, the latent image in the intervening years, I would tend to opt for a development process aimed at stronger contrast compensation (e.g. two-bath process à la Diafine, Emofin), so that you can lift the shadows without causing too much damage to the highlights. That said, it isn’t necessarily wise to use a developer you’re not yet familiar with for a one-off job like this (which, of course, I don’t know in your case). If I were you, I’d also keep in mind the option of simply
sending the film to our host in Berlin and having it developed there, i.e. in the fairly balanced A49, perhaps with a push of 1.
The above, however, assumes optimistically that a latent image is still present at all. Generally speaking, in such cases I would set my hopes for usable results a little lower. There are too many uncertainties at play. Who knows, for example, whether the loaded Clack hasn’t perhaps been sitting on a living room shelf for 20 years, with the sun shining on it every day and continuing to provide exposure to the film, or whether the gelatin hasn’t perhaps been rotting away in a damp cellar.
