The reason the mark is unusual is that it was only required for Belgian guns proofed for about 6 years, over 100 years ago.

Although I found it in Lee Kennett's work on foreign proofs, I'm sure he lifted it from Baron Engelhardt, who was probably the first to make the information available in English--and whose work Kennett was updating in a series of articles in Gun Digest (mid to late 70's), approximately 20 years after Engelhardt's articles had been published (also in GD). If you have the articles from both Engelhardt and Kennett, that pretty well covers foreign proof and proofmarks until about 1980.