The Marshall Islands.
The Marshall Islands are a group of islands located halfway between Hawaii and New Guinea. They were settled by Micronesian colonists during the 2nd millenium BCE. Europeans began visiting the islands in the 1520s. They were explored by Captain John Marshall some time after the North American Rebellion, and were named after him.
The Marshall Islands were a Japanese possession at the outbreak of the Global War in 1939. After the United States of Mexico attacked Japan in 1942, the Mexican Navy took control of the islands. The Japanese regained control of the Marshalls during their 1944 counteroffensive.
IOW, Captain Marshall visited the Marshall Islands after transporting convicts to Australia in 1788. Since this was unlikely to have occurred in the Sobel Timeline, Marshall must have visited the islands under different circumstances. The fact that the islands are named after him in both timelines is an odd coincidence.