orry, that variable is obsolete (although it should still work for
now).  I should have pointed you to the modern version instead:

org-list-allow-alphabetical


Note the caveat in the doc:

,----
| This variable needs to be set before org.el is loaded.  If you
| need to make a change while Emacs is running, use the customize
| interface or run the following code after updating it:
|
|   (when (featurep 'org-element) (load "org-element" t t))
`----

I tried it and it works:

  a. foo - press M-RET
  b. bar