That plaque was one of two that were originally glued onto each valve cover.
They were known for falling off. Someone in a distant time had one and figured it would look good stuck on the rear crossmember. Mounting is usually done with a couple of screws, bolts, whatever.
Originals appear from time to time on eBay. The vendors have access to a reproduction item, which you can buy direct from the machinist for $35:http://www.ipsco.org/Pantera%20Parts/plaqueblack.htm
There is a bling, billet version that incorporates the rear sway bar mounting caps. Vendors get about $250 for it.
I bought mine direct from the machinist for $170:http://www.ipsco.org/Pantera%2...20with%20Plaquer.htm
This is the same machinist, Mark Johnson, that I had fab up the replacement billet gas caps some months ago. View his home page at:www.ipsco.org