It's always tricky to identify lighthouse on the old painting due to it could be demolished, rebuild or painter could be not very accurate in the details of construction. Anyway let's assume that it still exist and looks relatively same. Before we move to Europe i would like to check have you considered following options in US:
Hi Pete! I did some research on this topic. There are number of "dunce cap" lighthouses in Japan (i.e. Hiroshima, Fujisawa, Hakata Kō - most of them were build after 1950s) I would say likely this picture is Yokohama South Outer Breakwater (built 1939). Here are some other pictures of it: LH Japan yamatae.sakura.ne.jp
It has slightly different configuration of windows (but this could be due to tower reconstruction) but general shape of the tower, breakwater and square building nearby - everything is very similar. As far as I've found in internet - American troops were landed in Yokohama after surrender of Japan so this fits to story as well.