B series number plates would have started around the early 60s and the series up to Es went through to about 69. With Cs in about 64, Ds in 65/66 Es were in 68.
In 69 the series started again with As - Bs were in in 71, up to Hs in late 74 etc - Ls in early 80s, Rs in early 90s.
I would say that if there was a B numberplate from either the early 60s or early 70s on a 2000s car then the number plate was most probably of some sentimental value to the previous owner and transferred from a previously owned car. If just a straight re registration it would have a new plate from the registration time.
Garry

