Humpty: you could be correct and I would like to know much more about post WWI Hungarian, Romanian, Russia and Yugo proof marks.

Mario:

To tell the truth I don't know what it is and some of the Belgian experts will have to chime in. I prefer numbers to characters. It may be pre-WWII or made at the end or just after WWII and that's the reason for the proofmark puzzle. I not sure exactly when the proof houses were in full swing post WWII. If you find any more info please post.

Kind Regards,

Raimey
rse