Raimey,
(Check your e-mail)
The "16" Is actually stamped KAL. 16. The "scribed lettering on the right tube near the forearm lug is FK or EK. Yes both tubes are stamped with S&S in oval near the flats.
Very interesting information above, thank you.
What happened to the original poster of this thread? I would have liked to have seen some pictures of his example of Kettner S&S.

Kind Regards,
Rob