Looking for Answers about Nikon Inc. MC-21 Extension Cord (10 ft.)?

Try asking a question about this on the page for MC-35 GPS Serial Adapter Cord. It appears to have a 10-pin pass-through, but I'm not sure if it supports all 10 pins. On that note, to answer your title question, the manual for the MC-35 seems to indicate that the MC-21 doesn't support all 10, as it must be attached directly to a camera, and not to an MC-21. The MC-35 isn't as cheap as the MC-21, but it's definitely shorter, and it could give you additional options for GPS attachment, as well. You might want to purchase some sort of cover for the D-sub serial connector, though, if it doesn't include one, and you don't plan on using that connector consistently.