I don't have any answer for you, but I found this in this thread (courtesy of @he-man40):
https://pinside.com/pinball/forum/topic/where-to-get-system-11-siderails-taxi
Long with the nail holes (Flash,High Speed,Fire! 1961-1987) http://www.pinballlife.com/index.php?p=product&id=3482
Long without nail holes (Taxi, Jokers, Cyclone,etc 1988) http://www.pinballlife.com/index.php?p=product&id=3488
Short without nail holes (BOP, Earthshaker,etc 1988-1990) http://www.pinballlife.com/index.php?p=product&id=3489