Simon;
Generator/Dynamo info is definitely not applicable...so don't look at that...but rather look at Alternator info for 123GT info here:
https://sw-em.com/123GT_Charging_System_Notes.htm ...as you may know, the 123GT also had an SEV Motorola Alt...so much of this info will be applicable**!
...and now that you're in the correct reference place, I expect the wire which was disconnected was from AMP Ind to 61 on Alt...this would cause AMP Ind not to function, but Alt and Reg to remain functional (in self-excitation mode)...now that you've reconnected this wire, non-rise in voltage when raising RPMs, as well as AMP Ind being continuously ON pretty much confirms Chg Sys is no longer working... I suggest you read and understand the theory of operation at link, then proceed to Troubleshooting Notes...
Good Hunting!
** Edit: In thinking about this a bit more...your '69 1800S with Alt sounds somewhat unusual...'69 was the changeover year to injection, and the later (140 style) multi-Fuseblock and Alternator (wiring diagram as here:
https://sw-em.com/1800E%20Wiring%20Diagram.jpg )...OR do you have the earlier harness (only 3 Fuses) but with an Alternator fitted (unusual combination!)? ...normally the earlier harness would have had a Generator/Dynamo fitted...I am not aware of the earlier harness/Alt combination from the factory, and as far as I can recall, I have never seen a wiring diagrams showing this combination... Please confirm configuration of this vehicle is as delivered from the factory and unmodified (as best you can)...if modified, current configuration must be very carefully established to allow valid troubleshooting...