I have discussed this with other contemporary makers, and the current thought is that the outline was stabbed in first, the the background scrapped away and finally the whole thing gone over with a V tool to clean up the outlines and leave the little CUT line at the base of the pattern...Given age, wear, and possibly sanding, or wooling and another refinish, and the result is the element is worn down and all that remains is the dark line......I do something that seems to work....once stained, restab the carving, then before finishing, mix some leaded linseed oil and black artist pigment, and with an artist brush, apply the mix to the border, into the stabbed or cut line, then finish the piece...seems to do the trick