The Experimental placard is due to the category in which the aircraft is certificated. IIRC P-51s are Limited category (a category that is no longer issued). Mustangs modified for racing are quite likely Experimental category. B-17s are Limited category, the B-24 I flew was in the Experimental category. Other categories are Restricted, Normal and Utility.
limited category aircraft are all ex military aircraft. The category was designed specifically for military aircraft. As far as I know, you can still have your aircraft placed under the limited category as long as it meets it type certificate. However, I don't know if they are still making type certificates for any military aircraft if that's what you meant and not individual aircraft. But then again, when I looked through all the type certificates for warbirds, I don't know if there are any left which already aren't there...
It seems the only two differences between Limited and experimental categories are the maintenance programs and limited aircraft are able to carry passengers and paying passengers through the LHFE program.
Our C-123 is experimental, but I believe the c-123s that were previously flying cargo in Alaska were all limited, however, I'm not completely sure of that.
I Know we can only carry members of our own organization on it. We even had to turn down being the drop plane for the Army Golden Kinghts once because of this when their plane went tech at an airshow.
