Are you saying the motor number boss is actually stamped 43US? If it is, then you have an issue. While sidecar geared U models were called US models, the S does not appear in the actual VIN.
South African Union Defense Forces (Army) Big Twin sidecar models were 41U, 42U, and 44U. They neither purchased 1943s nor received 1943s through lend-lease. However, Their 1942 contract was delayed til late in the 1942 Harley-Davidson production year so that many have 42U VINs with 1943 bottom numbers. The same way late 43WLCs have 1944 bottom numbers.
Your U may be ex-S.A. Army but either the motor was replaced with a 1943 motor from another source or, if the VIN boss is showing 43US, the VIN has been altered or the case replaced with new ones and then stamped incorrectly. What is the serial number on the sidecar chassis and what is your motor's bottom number?
One other thought about a VIN bossed stamped 43US, an original stamped 43U might have had enough space after the U for someone to cleanly insert an S into the VIN without disturbing the rest of the number. That might give the appearance of an unaltered VIN while, in fact, it is altered from the factory original.