how to pass /E:ON argument to cmd.exe using exec (Windows)

For a change of pace, this will not be a Tcl 9 question (I'm using 8.6.14).
Using tclsh, why does this succeed:
     exec -- cmd /C dir
but this fails:
     exec -- cmd /C /E:ON dir
with this message :
     "The filename, directory name, or volume label syntax is incorrect."
I'm guessing that "exec" is treating /E:ON as a file spec (the colon maybe?) and then changes it somehow.
A followup question is how can I see what string is being presented to to Windows?
Thanks for answering.
-mike