Hi shuter,
I've recently been transitioning into trying to mark all adapters with "+" and "-" symbols on the sides of the end-caps to make cell loading easier, I apologize if I missed your original adapters. In the "slot" that has "copper sheet over screws on both ends, the "+" sign should be at the bottom of the adapters, and the other 2 slots should be the reverse (the slots with hex nuts as contact) with the "+" sign at the top of the adapter.. A black permanent marker works great and is quick and simple. The parallel adapters I sent out recently should have all of the top marked with "+" signs, and all of the bottoms marked with "-" signs...
However, assuming no polarity "signs" on the adapters at all, a quick visual inspection will reveal the electrical path. Looking at the end caps, on serial adapters, the copper "link" is between 2 points on the end cap, and shares no electrical path to the "nut" that serves as the final contact through to the outside of the end-cap. On the parallel adapter, you'll notice that all of the points on the end cap are electrically connected.
I hadn't taken your situation into consideration, this brings to light some extra things I should be doing to improve the quality of the product. I need to make a point to maybe mark all parallel adapters with the letter "P" or something.
Hope that helps, and thank you for asking! If you have any other questions or concerns, let me know
Eric