Here's a link to firearms made by Janssen Frères.
The small 410 sxs shown first up has the JF marking on the bottom of the bbls right in front of action flats.
One tube is readable as JF. The other has a slight mis-stamped mark. Still readable but you can see how another slight misalignment while stamping could result in the
mark seen on the MH.
Or at least it kinda looks like it to me!
https://littlegun.be/arme%20belge/artisans%20identifies%20i%20j%20k/a%20janssen%20freres%20gb.htmThere was an Adolphe Janssen as well as the others.
A high grade maker and had one of those "I make guns for the Royals" cards in his wallet.
But his maker mark was just a plain 'AJ' as you may well have guessed.
Sometimes inside an elongated diamond. Other times with a small crown over the plain letters.