Definitely by 1977 when the UFO was published, knife-edge flight was in practice. The article specifically discusses features of the plane intended to improve knife-edge flight, like the increased side and fin area (over the Dirty Birdy).

I also see the SK as a kind of "stopgap" design. The enclosed engine and addition of retracts was clearly intended to cut down on drag for faster flying speeds. But, Bridi must have seen it as having reached the limits of the Sun Fli/Kaos lineage; the DB is a total departure/clean slate.

David