I'm sorry Peter, but the gun is of lower quality and tens of thousands were made in Belgium before WWI for export. It is quite unlikely to have any relationship to Delcour-Dupont, and certainly not Francotte. The 'D' may be one of the Dumoulin concerns.
http://www.littlegun.be/arme%20belge/artisans%20identifies%20den/a%20dumoulin%20gb.htm The barrels are Twist and for some reason the browning was removed, possibly in preparation for refurbishing and re-browning. I've seen the 'AF' before, and it likely is the mark of the (unknown) tube maker.
As a piece of family history it is of course priceless, but under no circumstance should you attempt to use the gun.
