If you check out his career figures, there is no ‘best match figures’ recorded for his first class career. As in ‘we have not been able to find what his best figures in a first class match across two innings are’
Seeing as Nash took one ten-wicket haul in his first class career, which was in a Test at Lord's against England in 1994, that would be his best match figures. Which source is stating Nash's best match figures are unknown?