According to our sources: from an article in Classic and Sports Car Magazine, John did not buy it new but purchased it in 1992, He owned his 507 from new.

Surtees acquired a taste for 50-series BMWs and in 1992 bought BEE 46.It had been sitting, largely unused but well preserved, in AFN’s private collection for 25 years, Dr Bee having returned to the dealer 10 years after buying the car to trade it in for a Porsche, of which AFN had since become a distributor.
Decades later, when Porsche bought AFN outright, the BMW was up for sale – and Surtees was tipped off.