That information on the custom transponder is incorrect. The other guy had the correct information. MRT does have a "pool" of transponder numbers that they use to create the custom transponder's 21st channel. They buy up broken transponders, extract the numbers and use them to produce the custom transponders. There is a small chance that there could be a conflict, but I think its pretty long odds from happening.
If this really bothers you, just get a used AMB transponder,
working or not, and have it cloned as many times as you want. In my case, I didn't have an AMB transponder, and I race with my son, sometimes in the same heat. I needed 2 different transponder numbers so I decided to get 2 of the cloned units. There are several people at my track who also use the MRT transponders and I haven't had any problems.
There is a really long and involved thread about this in the micro forums.
http://www.rctech.net/forum/micro-mi...ansponder.html
Terry from MRT has posted all the information on this stuff, but you will need to read lots of pages of posts. The thread is a bit old, and it started before the 20+1 custom/clone setup, but keep reading or skip to the end and read backwards. Somewhere in the middle, he announces the 20+1. I read the entire thread and asked Terry questions before ordering. It took me a long time to understand the differences between the two.