There's not a part shown in the diagram for body-mounting a coupler. It doesn't look like the assembled versions omitted any parts from the kit.
"Step 2: Installation of Bolsters, Center Sill, and Hopper Gates
...
If body-mounted coupler boxes are desired, use the bolsters with the integral coupler box lids (i,k) and remove coupler boxes from trucks...."
Also:
"IF YOU FIND THAT A PART IS MISSING OR DEFECTIVE, OR IF YOU SHOULD LOSE OR BREAK A PART, PLEASE CONTACT US FOR A REPLACEMENT"
For those of us that were too late to buy kits and bought RTR cars with truck-mounted couplers, it follows that there were no "missing" parts for us to ask for.
For anyone that bought these kits with these instructions, IMRC customer service should have addressed this, whether by clarifying any inadvertent references to parts that don't exist, or alternatively, by honoring any missing parts requests to those that wanted to use them per their own instructions.