Hi James,
These letters are a bit larger and in a different font than the usual inspector cartouches and stamped in a different location than the usual inspector cartouches.
The location seems to be about the same on the other arms mentioned as well. I begin to wonder if they are a collection or similar marking rather than a unit marking. However, I will be very interested to hear any other theories or information or of other examples.
Thank you for taking time on this.
Dušan