Last time I was in Malden, MA they had several 27 mhz newer (on-off) units on the shelf.
There are 5 versions of the 27 mhz transmitters that I know of.
older 2 track version.
10 chanel 10 frequency units as follows:
original where the forward and reverse buttons are real
updated version where the forward or reverse just reverses direction. This was done due to George S request
updated updated where forward is forward and reverse just does a direction change This was done as some did not like George S new request.
newest version computerized with on-off button.