Martin did have a relationship with Bill Frieder, and was a known entity in the Michigan program in the (mostly) late 80s. Don't know where 1981 comes from, because that's far earlier than any date I've ever seen. Martin was mostly a Soutwestern guy, but between Antoine Joubert and Jalen Rose, Michigan didn't get a single kid from Southwestern. Yet Martin was a guy to know and keep in your good graces if you expected to recruit Detroit, as was Perry Watson.
The reason why the April 1992 date is important isn't because of the "booster" designation, but rather the exact moment when a paper trail for benefits between Ed Martin and the Webber family could be made. Michigan gave Ed Martin two rooms in the team hotel. Martin gave one of those rooms to the Webbers. The moment the Webbers took possession of the room, Chris lost his eligibility. Hence why the 1991-2 season was only vacated from the Final Four onward.