(clos) additional properties to slot definition

http://mop.lisp.se/concepts.html says:

An implementation is free to add additional properties to the canonicalized slot specification provided these are not symbols accessible in the common-lisp-user package, or exported by any package defined in the ANSI Common Lisp standard.

with example:

(defclass sst (plane)
     ((mach mag-step 2
            locator sst-mach
            locator mach-location
            :reader mach-speed
            :reader mach))
  (:metaclass faster-class)
  (another-option foo bar))

But when i try:

(defclass a () ((x my-option 123)))

sbcl compile it with error:

Invalid initialization argument:
MY-OPTION
in call for class #<STANDARD-CLASS
SB-MOP:STANDARD-DIRECT-SLOT-DEFINITION>.
[Condition of type SB-PCL::INITARG-ERROR]

So the question. How can I add additional properties to the slot definition?

I'm no clos expert, but wouldn't you need to use a different metaclass?