Hi Trinity,
Yes, before 1992 the blood supplies weren't tested for Hepatitis C, so it's more than possible that could have been how you became exposed.
Blood transfusion, IV drug use, working in the health care field -- are the more common modes of exposure, but there's a certain per cent of us that don't have any idea how we got it.
As to no symptons, the first sympton a lot of us have from this disease is a state of shock when the doctor tells us we have it :) That's not to say that Hep C is asymptomatic, but with many it appears that way, even for long periods of time like in your case. Probably lucky that the "Porphyria" gave it away or you might not have known for some time.
Anyway, the biopsy will tell you a lot about what has been going on with your liver all these years. Hopefully you'll be one of the lucky ones without too much damage, regardless of how long you were exposed. I had Hep C for close to 35 years before I treated, and probably was somewhere around stage 3 by the time I got started. Let's hope you did a little better than me, but even if not, things have never looked better in terms of treatment protocols, new drugs in trials, etc.
Let us know how your biopsy goes. Smart to see what it shows before making any commitments to treat. One thing at a time.
All the best,
-- Jim