Check this out. Seller claims that local dealer in Michigan was known to have sold three of these. Looks professional. E-Bay item 4634535469. Sorry, I do not know how to attach to make it easy.
here it is
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=4634535469&indexURL=15#ebayphotohosting[/url]
I do recall aftermarket rumble seats in the late 60's, but first time I saw one or heard about one on a Mustang. Be kind dangerous for the passangers since they are sitting over the gas tank.
I remember Mustang Monthly doing a feature on one of these a few years ago. I believe it was a Playboy Pink '69 convertible with wire spinner hub caps.
In 1969 there were 10 1969 Pink convertible Mustangs with Rumble seats made for Benton Harbor Michigan for a big parade celebration that they were having.
I talked to a guy that had one at a car show we were at a couple years agao. He had lots of documentation and articles on them.