You have to start with the original DA 9mm produced by S&W: M39. This was a single stack gun which was inspired by the Walther P-38. While it discarded the Walther's locking block (adopted by Beretta) in favor of the more traditional modified Browning tilting barrel lock-up, it retained much of the Walther's safety features including a firing pin block and decocking safety.
The M39 was modified to accept a staggered magazine which was adopted from the Browing P-35. The new gun was the M-59 pistol, from whence the 6906 evolved (different materials, reengineered trigger for better pull, comestic differences which enhanced the ergonomics of the gun).
Class dismissed?