The rkmmodel only has one "gettable" attribute: rkmmod_currval, which
makes processing it easy.  The om_get message looks like this (defined in
<intuition/classusr.h>):

    struct opGet {
        ULONG MethodID;     /* OM_GET */
        ULONG opg_AttrID;   /* The attribute to retrieve */
        ULONG *opg_Storage; /* a place to put the attribute's value */
    };

When the rkmmodelclass dispatcher receives an om_get message with an
opg_AttrID equal to rkmmod_currval, it copies the current value
(rkmmoddata).currval to the memory location opg_storage points to and
returns a value of TRUE.  The TRUE indicates that there was no error.  If
opg_AttrID is not RKMMOD_CurrVal, the dispatcher should let its superclass
handle this message.

The rkmmodelclass dispatcher can take advantage of the fact that the only
"gettable" attribute available to an rkmmodel is rkmmod_currval (the
attributes defined by modelclass and icclass are not gettable--see the
Boopsi Class Reference in the appendix b of this manual for more details
on which attributes are "settable", "gettable", etc.).  If opg_AttrID is
not RKMMOD_CurrVal, the rkmmodelclass dispatcher can return FALSE,
indicating that the attribute was not "gettable".

If the rkmmodelclass dispatcher comes across any other messages besides
om_new, om_set, om_update, and om_get message, it blindly passes them on
to its superclass for processing.