Fids are generally of a hardwood and are used for splicing laid (twisted) line
and for other marlinespike work, like stretching a grommet or cringle.
A 'Swedish fid' is used for splicing laid line and has a polished metal channel
that tapers to a point and a rounded hardwood handle on the back end. When
pushed through the lay of a line, it leaves an open channel through which a
strand can be fed.

Rich wrote:

Friend has been given his grandfather's rigging knife. Is the pointed
tapered tool for untieing knots and opening the lay of a line called a fid?
What else is it for? On the side of the knife is a tapered slot routed
into the wooden handle and then overlain with steel with the matching slot
milled into it. The slot is not undercut . It is about 3/16" deep, about
1/4" wide at the large end and about 3/16 to 1/8" at the other. Whole slot
is about 1 1/2" long.

Is the slot another tool? What is it for?

Thanks,
Richard