all 75 and 164 twinsparks had VVT.
The early 164 TSs used the Motronic ML4.1 engine managment systems while the later ones got the newer Motronic M1.7 system, but even the earlier ones had VVT.
I think I can see the actuator in your photo, the black cylinder hanging off the front of your inlet cam?
[edit - I see you've found that already]
Had you swapped the leads to their correct spots prior to starting it this morning?
A pop in the plenum could be from a spark plug firing while that cylinder's inlet valve is still open during cranking - when I've had that happen I wouldn't describe it as a "little" pop though.
It could be a leaky injector, overnight it slowly dribbles a small amount of fuel so one cylinder is very rich during startup if it's sat for a while.
A bottle of injector cleaner in the tank might help, but if the injector is worn then it won't make much difference.
Or to test the spark, you could try disconnecting one of the HT coil leads before starting. If there's no pop then try disconnecting the other coil instead the next morning.
With one coil disconnected your engine should still run fine and smooth if the leads go to the right plugs.
If they're not in the right spot, it'll either run very rough or not run at all.
By disconnecting one coil it makes it a lot more obvious if there's a problem, because the twin igniton systems work as a redundant pair - you probably wouldn't even notice if one of your coils died while you were out driving as long as the other one was still working...