Here is a video which is an overview of how a Caleffi brand hot water system is installed.
At about 1:17 in, there are a couple shots of a brass coupler connecting one panel to another. Right after that, they show the spare end coupler being capped-off.
I think we want something similar to that for our refurbished panels.
Later in the video, it also shows filling the system with antifreeze. I think he goes through about 5 gallons total, for a two panel system with the panels on the roof, tank in the basement, and pretty large diameter hose connecting them (1"?)
Also, it looks like the flow rate was adjusted to pump 2.0 gallons per minute.