Hi Stu,
The responsible thing to say is get a mylaps transponder. Best is a hybrid as its backwards compatable from RC4 to the older timing system (RC2 and RC3).
The number you get if you buy new is allocated only to you but you will need to move it from car to car.
Or
You can get an MRT one they are clones and use cloned numbers from old units so 1 transponder has 11 numbers to chose from ( 1 main and ten alternatives numbers, in the VERY rare situation that you get a number conflict).
If you phone MRT they can do you 2 (or however many you need) transponders with the same main number so in affect 1 number for all your cars.
If you get MRT get the newer RC4 compatable ones that covers you for wherever you wanna race.
PS did i mention they are also cheaper and support is second to none!
So for me its a no brainer.
(All mine and Toms car are equiped with small black, gold lettered beauties.
)
HTH's.
Shiny