Adrie,
I made a drawing to show my point.
It’s a terrible drawing, but still shows what I mean.
Skinny lock mortise, but maybe a bit thicker at the nose, like on Dave’s guns.
Also, the wood on the side of your tang is flat. In other words, it is straight across all the way. It should angle down. If you fix the wide lock mortise, then this will fix itself. The wood on either side of the tang should angle down fairly sharply, in a straight line to the lock mortise.
I hope I have shown this in the drawing. The picture is from the rear of the gun, looking at the tang.
Here is my terrible drawing:
Gonny,
I hope I have made this easy to translate!
Cheers,
Norm.