Light shines only through the text part, giving them a cool look. There may also be other differences in the art, colors, etc, but I haven't put it side by side with a production playfield to truly pore over the details and spot the differences. I have no idea how many of these they made, but I have never seen another one. I've always kept it as a collectible, but it could also actually be used in your machine to make it one of a kind. It has a John Borg (I think) signature on the apron area, as well.
The item "Metallica pinball machine NOS playfield PROTOTYPE Stern SUPER RARE New Old Stock" is in sale since Wednesday, March 31, 2021. This item is in the category "Collectibles\Arcade, Jukeboxes & Pinball\Pinball\Replacement Parts". The seller is "bobbyconover" and is located in Edmonds, Washington. This item can be shipped to United States, Canada, United Kingdom, Denmark, Romania, Slovakia, Bulgaria, Czech republic, Finland, Hungary, Latvia, Lithuania, Malta, Estonia, Australia, Greece, Portugal, Cyprus, Slovenia, Japan, China, Sweden, South Korea, Indonesia, South africa, Thailand, Belgium, France, Hong Kong, Ireland, Netherlands, Poland, Spain, Italy, Germany, Austria, Bahamas, Mexico, New Zealand, Switzerland, Norway, Ukraine, Croatia, Chile, Colombia, Costa rica, Panama, Trinidad and tobago, Guatemala, Honduras, Jamaica, Barbados, Bangladesh, Bermuda, Brunei darussalam, Bolivia, Ecuador, Egypt, French guiana, Guernsey, Gibraltar, Guadeloupe, Iceland, Jersey, Jordan, Cambodia, Cayman islands, Liechtenstein, Sri lanka, Luxembourg, Monaco, Macao, Martinique, Maldives, Nicaragua, Oman, Peru, Pakistan, Paraguay, Reunion, Viet nam, Uruguay.