HondaNutRider:
I think you are quite correct that this was a rubber plug. Probably the chain guard to shock clearance was so close that they wanted a little bumper between the two. Since it is well hidden behind the shock, I think I'll just glue on a little patch of oil proof rubber.
Thanks for the help.
Maybe if you're in London we'll meet one day. I'm at Western pretty frequently, twice a week. Don