I bought the CM15A, and a bunch of other X10 stuff, used so I don't know it's age either but it has (what is labeled on the main board as) an "RF DAUGHTER" board mounted on the main board close to the connection to the outside antenna. I don't see any separate receive or transmit wires or antennae. This daughter board has a single chip mounted on it and a single wire runs from it to the outside part of the antenna. That single wire makes a single loop inside the case with maybe 4" of wire in that loop. It is glued to the main board in three places so I guess that glue (looks like hot glue?) would have to be removed to allow stretching it out beyond the end of the antenna tube (if that's what you were referring to exactly).
Adding BTW the 4" of wire inside the case to the antenna length of about 5" gives 9" which would be the length of a 1/4 wave antenna(?). I wonder though how much range increase would be gained by pulling the wire out?
OTOH, it looks pretty easy to mount an RF jack and external antenna on it which would be a cleaner solution than an outboard wire stuck on all the remotes.
I should have mentioned I'm in Canada so I'm guessing the RF would also be 310 MHz(?).
Also tried a 1/4 wave external wire on the remotes - reduces range somewhat over the 1/2 wave but still more than enough to get the 60 feet or so from inside the house to the garage (I've also noticed it's prone to dead spots which might explain I guess some of the inconsistency folks seem to be getting with respect to experimenting with antenna mods).
But hey, this experimentation with the hardware is as much fun as the other stuff that goes with setting up a system.