my dealer told me about this yesterday. he said he did not know exactly why but it could be as simple as an air filter issue and Ford will hold production on all vehicles. Again, I have absolutely nothing to back this up, just stating what I was told. a few months back he told me the first car they got would be race red and they'd have it by May. now he says it will be later due to the "hold" he told me he could only get a partial vin number and once he can get the full vin he can give me the details on what options it has. Not sure how true this is
Material hold usually indicates a 3rd party supplier is having a shortage of supply and not able to provide enough parts on time to meet demand. There are other reasons such as a part is found defective and getting updated but most of the time it is just supply and demand.
It is very common on a lot of vehicles, just gets more visibility when it is a GT500 or a ZL1 where people are tracking every single issue or event.
"Material on hold" usually means that some supplier isn't able to deliver parts on time, or a defect has been found (usually accompanied by a recall for vehicles already shipped). That doesn't seem applicable to the Shelby though.