The sleeves are made with gussets like a shirt, and are gathered into the arm holes.
The gussets are also 15 deep, the width being 5, the same as the bottom.
Sew this, then take the other side of bag and sew to the gussets.
The gussets, facings, etc., are cut from the waste leather in the thumb opening at the same operation.
It was a problem—simple to the speakers—of gussets, ...