Thanks Tony. Any tele lens with a 10" dia optic is going to be a bit of a beast. If you can get some photos of it, I'm sure it would help anyone here in trying to identify it.
T2 mount is a very versatile mount, but now we only tend to see a few accessories using them.
The size of that front element @ 400mm focal length suggests a very bright lens indeed. A quick guesstimate has me thinking an aperture of something like f1.0 when it's fully open!! wow!
(sounds like movies camera lens territory. arriflex, panavision etc?)
Next step I would definately recommend dropping a line/email to a
specialist Nikon retailer..."Grays of Westminster" is a good place for example...
They'll probably need all the details from the lens barrel etc, maybe including serial number. they might even need a photograph of it too?
I've had a rummage through my copy of Hove 'Blue Book', but there's nothing matching your lens. similar lenses (assuming a 35mm film lens) estimates vary quite massively between £1000 through to £6000 for a secondhand unit (sorry to be so vague)
hope this helps you Tony?