Hi Ralph:
These are as outdated as spark ignition. They are available occasionally, but at highly inflated prices and are mostly clapped out.
I can highly recommend the bandburner electric timers from BMK. The only drawback I can find is that they have to be started from hand launch of model rather than starting on release from towline.
https://bmks.co.uk/products/bmk-band-burner-e-timerI use them in my E 20 Blizzard model due for publication in June Aeromodeller.
If you really want to go elite try RDT (radio released DT)
https://bmks.co.uk/products/bmk-remote-dt-b1john O'S