RCA often uses a macro for functions like power off. That macro sends the command you want mixed in with a number of other commands. Learning remotes are generally bad at learning such macros.

I hope using the common EFCs (as I described in revious reply) works for you. A more difficult approach is to make the 2103 learn a fragment of the macro by moving or covering/uncovering the original remote quickly during the learning process. It probably would take many tries because usually it won't learn and when it learns it will usually be the wrong fragment. With JP1 that might be a practical approach because you can at least download and see what fragment was learned each time it learns and confirm you're on the right track and maybe extrapolate to the right answer. Without JP1 it probably is far too much effort for only a moderate chance at final success.