The Smiths application list for that year says: "5330/111" to match the "SSM5001/03A" speedo.
The 5330/111 gearbox is listed as 21-10 (or 2.1:1) ratio so would be correct for a "1600" turns per mile instrument.
However, the BSA parts book for '66 lists the drive gearbox as: "1.25:1" (BSA part 19-9217) to match speedo SSM5005/01. So there could have been an early or mid-season change, as a 1.25:1 drive gearbox would almost certainly need to be used with a 1000 turns per mile speedo?