Here's an interesting refoam job on a 2231A as far as I can tell. Looks like the normal foam rot was replaced with a circular ring of sheet packing foam (the kind used as a liner or maybe a large envelope). It looks properly glued to the back of the cone. From the front, looks like the foam was painted or Sharpie'd black but it looks more salt 'n pepper. Might even work?!
http://cgi.ebay.com/VINTAGE-JBL-PRO-...QQcmdZViewItem
Here's the pics.
Regards,
Bart