OK, just as update. Old blond hippity hopping Bunnie 's list is more comprehensive and much more easier to read and understand than mine. Everyone say, "Thanks, Bunnie!":

System-------SLT------------------DTMF
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
DK8----------QSTU-----------------QRCS

DK16---------KSTU1/KSTU2----------K<>RCU
DK16-w/Exp---KSTU1/KSTU2/RSTU-----K<>RCU
<> = Any digit 1 to 5 will work fine, 2 to 5 backwards compatible

DK16e--------KSTU2----------------K4RCU/K5RCU
DK16e-w/Exp--KSTU2/RSTU-----------K4RCU/K5RCU

DK40---------KSTU2----------------K4RCU/K5RCU
DK40 w/Exp---KSTU2/RSTU-----------K4RCU/K5RCU

DK280--------RSTU-----------------RRCS-4/8/12

DK424--------RSTU-----------------RRCS-4/8/12

DK424i-------RSTU-----------------BRCS-4/8/12
Note: Any version of PSTU or RSTU will work in place of listed RSTU
-----------------------------------------------
CTX28--------GSTU1a---------------INSTALLED

CTX100-------RSTU-----------------AETS1A IF ACTU1A PROCESSOR
CTX100-------RSTU-----------------INSTALLED IF ACTU2A PROCESSOR

CTX670-------RSTU-----------------INSTALLED, 1st 4 port activation included