Most aren't compatible. shimano will typically marry with shimano hubs, but hope etc...wont marry shimano, etc...My
knowlwdge rests with shimano -89 to present and, with 8sp like stx-rc-deore-XT etc are similar but have slight differences with grease guards and other o-ring seals
For example, the splines at the rear of shimano freehub bodies are the same on 90% of shimano freehub bodies. I think RM30 and so-called 'silent clutch' were diffe-rent. some other maybe too.
But, as daft as it sounds i could remove the FHB from my exage HG50 hub and install it to my XT 750 hub, now, the obvious differences apply there, suddenly i'm 7 and not 8 sp and i would have to swap spacers, seals and all sorts of other bits and pieces, just to make it work but, it would work as the spline interface is the same.
Are we talking shimano here?
If so, there is a shimano tech pdf for matching up compatibilty with hubs. Also, if you stay close to existing (pressumed knackered) hub, then you should be ok, although swapping some spacers and seals may be necessary to get it all installed nicely.
From experience, ritchey and shimano FHB's are different. Hope is in it's own world(good stuff though).
Let us know what you have in mind, i'm sure some useful links will crop up.
Ride yorkshire on an old bike.