Native American tool?

Could anyone I.D. this native american stone tool by description? It was found near the Montrose Ark. area approx. in the 1950’s.
It is a small black stone artifact of fine craftsmanship and polished very smooth. It is 3 1/4 inch long by 1/2 inch deep bye 7/8 inches wide. Both ends are v-notched. The bottom is totally flat from end to end. On the top middle area is a flat surface the size of a man’s thumb, from this flat surface it tapers gradually down toward the v-notched ends of the tool. In the base of the v-notches it appears that there are thin, shallow wear marks as though somehting string like was drawn through the notches.
In 1972 an amature archeologist suggested that it may have been a “bow string dresser”.
I have never heard of such a tool now have I ever seen another like it or even similar.
Note: It is definately not a net sinker or a game stone. It is clearly a tool to be used by hand. Not sure if it was from the “missippian period or the “poverty point” period.
Any info would be most appreciated as we have tried for years to I.D. it.