*PLUMBERSSTOCK EXPERT* responded:
Hello! Thanks for visiting PlumbersStock.com! According to Mars, the common connector would be the one with a C next to it. This is a dual cap, so the common connector will be the center. If you're ready to order this MARS 12190, just click "Add to Cart" and put your postal code and you're ready to go. It is our pleasure to serve you. For our latest offers and news about new products, you can subscribe to our free newsletter. Just register using your email address. Thank you for choosing PlumbersStock.com as your online shopping buddy! Have a great day!