I did some googling before I created this post and saw that graphic. That graphic initially convinced me this wasn't an antenna which led me to create the post. But it's a generic graphic and doesn't show all customer options.

Reasons it's not a doubler: It's about 0.25 inches thick (or thicker). 777 skin thicknesses are typically .060 to .100. A doubler is typically no more than one gauge thicker than the skin it's repairing. If a doubler is too thick it creates a hard point and draws extra load. It's simply too thick to be a doubler.

The fasteners are spaced too far apart to be a load transfer joint. Probably more than 2 inches? Load transfer joints have a typical fastener pitch of 5 times fastener diameter. Or 0.95 inches for a .190 fastener.

This antenna is attached with a single row of fasteners all the way around (26 in total). Doublers are typically attached with three rows of fasteners. Multiple rows of fasteners spread out the load transfer and have better fatigue performance.

I've seen dozens of doublers and even designed one. This is simply NOT a doubler.

Finally, the friend that confirmed this for me has access to the Boeing drawing system. He said the part is named "satcom" under the part description.