I recently picked up a 1992 Santana Vision with an unfortunate mix of components including a 9-speed Deore shifter mated to a 7-speed cassette on a Hadley hub. I'd like to move to a 9-speed cassette, but I have my doubts that it will fit on this hub. Adding to the uncertainty is the fact that the wheels are mismatched, so I'm not sure if either is original, and there is no model number on the hub.
I did manage to find similar discussions in the archives at tandem@hobbes, and it looks like the cassette body can be replaced and mated with a Ritchey lockring to ensure full thread engagement. Does that sound like a reasonable upgrade with this hub?
My guess is that is actually a 8-9-10 speed compatible hub. I don't think Hadley made hubs for Santana that far back. Most likely a replacement wheel, and a nice one at that! I have four sets of wheels for my Santanas, all with Hadley hubs. Never had a problem, and the ease of disassembly with just a 5mm Allen wrench is great (just like Phil Wood uses). From your photo, it appears that there is a spacer between your seven-speed cassette and the back of the freehub. This is used when you run a seven-speed cassette on a newer freehub body. Just remove the spacer and you should be good to go with a newer cassette. If you use a 9-speed cassette you won't even have to change your shifter from the sounds of it.
BTW, the black thing on the left side of your hub is a thread-on/off cover that covers the threads for a drum brake (or disc brake with adaptor).