The S70 is the best of the BSF break-barrels. It has a long barrel which makes it less jumpy than the B55. I think the stocks are walnut on some of them, or maybe all of them. You might get £200-£250 for it in .177. All BSFs are quite rare.
The rifle is the first air-rifle which could hit or exceed 12 ft/lbs in .177, there was nothing until the Feinwerkbau Sport came out in the early 1970s to match it in terms of power. They are more agricultural than Weihrauchs but lighter and less ponderous. The huge amount of preload and a powerful spring mean it is quite a difficult rifle to master, but they can be quite accurate after lots of practise.