Hi Mike,
Glad to get your message.
You need the inside O-ring of the hardline 14mm OD fitting, right?
So BP-ML14-O10-BK is the correct one.
Also, you can use the tool BPTA-ORFT14 to put the Oring into the fitting easily.
https://shop.bitspower.com/index.php?route=product/category&path=59_128_140