On the 40 foot RBBB box car I need to replace the EX coupler. Which one do I need?
EZ coupler.
Replace it with one of the same configuration; meaning, shank length and position of knuckle, ie: center set, under set, over set. That is, if the original's coupler height was good and at the right height. Replace it with one that has a knuckle spring. These are EZ Mate Mark IIs.
One further thing; when you get new couplers with the spring in the knuckle, before installing them, put a teeny tiny drop of CA (Crazy) Glue GEL on the spring at the end where it meets the shank. This will help keep the spring from popping off and still allow it to flex.