Very nice too. It just shows they are still out there just waiting to be found. At least the box label appears in better condition than my remaining example.
My own Mk.11 Target, serial: 14154, doesn't have any caliber markings either, nor does it have any letters under the frame. I believe these could have been inspection letters, but JM will probably be able to better comment on this.
I thought that as a 'Target' model the calibres would all be .177, as the examples I've had, and the one I still have, are all .177, but as Trevor has a .22 it appears I am mistaken.