Jim, I hope you don't mind this sorta off topic posts in your thread...
The 16 mm ID tube is on pump inlet with no reducers, the reducer is on the other end of the tube, on the flow indicator output barb. The copper elbow is completely inside the 16 mm ID tube, and the reducer is made by simply putting one piece of the 12 mm ID tube on the barb itself, than 16 mm ID tube over it, and with a good clamp...voila!
One can also use the copper elbow as a reducer by simply sawing off one end of the elbow to fit the 12 mm ID (little stretched) tube over it, and the wider side of the elbow fits nicely into the 16 mm ID tube (think that outside ID of that elbow is 16.5 mm or close to that...)
Cheers!