From reading online, the mercury fuel connectors came in two sizes. The smaller one is supposed to fit the Scott/McCulloch fuel barbs.

Here's an example at the beginning and end of the video:


Here is the cheapest I've found it so far. They are getting expensive:
http://www.ebay.com/itm/MERCURY-BOAT-FUEL-LINE-CONNECTOR-30635Q-3-1971-AND-OLDER-/280878944270#vi-content