The Hub shell has a date of production on it .. my Mk 2 had a 94 hub in it [its likely a Mk2]

Sachs Made their 1st 3by2 6 speed .. that introduced the 2 cog 13, 15 pair on the Driver .

That would have the shifter/derailleur under the right chainstay (thats Not there) thats A Mk 3

so, Agree, Its the S-A 5 speed hub - 2 indicator - chains 1 on each side of the Hub.

left one makes the narrow 4th & 2nd, or wide 1st, 5th gear difference shift . 3rd is in both


I went with a Schlumpf Mountain drive to have 6 speeds . the low range uses the 3 speeds a second time.
a real Mountain climbing setup.

Mk4 is the current Frame Hinges are cast tube brazed in , Mk2 used a Fillet Butt Brazed Joint
forged, flat Hinge plates.