Nice find!
as long as they are big enough to clamp in a vice, you can start your groove with a small rat tail file. When you file a pivot in one of these blocks you then need to give the groove a quick brush out with a wire brush as particles can spoil the finish when you then burnish it. If you want to make a bigger block you could inlay the lignum into the edge of a cheaper wood block as its only the edge you need.
Hope that makes sense?
Paul.