RHD (Right Hand Drive) would mean your car was assembled in either Australia or Europe.
Your Firewall O (or 0) A731123. C73 is the code used for Custom Deluxe Fordor sedans assembled in the US.
Your Motor/Chassis O or (0) 17849. The motor # has nothing to do with the Chassis #. US assembled cars refer to the Frame #. On US cars (0) zero means 1950 model year car. B stands for V8 engine. The next in the sequence is 2 alpha characters for the Assembly Plant. The 17849 wuld be the sequential assembly number for that assembly plant for that year.
The best information source for US assembled cars is: The 1949-50-51 Ford Book A Restorer's Guide from The Early Ford V-8 Club of America www.earlyfordv8.org. It's a great club to join with many local and international chapters.