If your v5c says 1975 the rolling Historic Tax rule came into effect yesterday.
Just changed my tax bracket and got my years ticket for nowt at the post office today.
It's a good feeling - mine came a year ago. After a year you get the reminder to re-tax, though. It still has to be taxed every year, which does seem a bit unreal. Still, at the price, I don't mind.
Mine was registers 76 but have a certificate to prove it was built in December 75 so while im off work tomorrow i shall be popping in to the post office with my logbook