G'Day Waidmannsheil,

Only a guess but I always assumed, it was where you placed "A Loop of Thin Rope", as a "sling" attachment point, for the firearm?

Further to this, prior to the availability of metal sling swivel mounts, and slings with hooks to suit, I assumed you placed a Loop of Rope (of just the right size Loop, or maybe a Hangmans Knot?), over the muzzle and down into this groove in the forend tip. Once in place there, and then slung over your shoulder, this loop would have the weight of the firearm on it, and with the angle of the groove in the forend tip, this combined, would hold the rope in place.

The other end of this piece of thin rope, was possibly wrapped around the "Pistol Grip" area, or some other part of the butt stock?

Hope that helps

D'oh!
Homer