If it helps at all, it should be interchangeable with just about any Harvard/AT-6 LH flap assembly.
The postwar RCAF Illustrated Parts Catalog lists 55-18001 as the LH Flap Assembly for the Harvard Mk II and Mk 4.
Also, the 1943 NAA Interchangeable Parts Catalog lists 55-18001 as being used on the BC-1A, BT-14, AT-6, AT-6A, SNJ-2 & SNJ-3.
I'm guessing it should be interchangeable with later variants, but don't quote me...
Good luck in your search!