Interesting, I'm not sure I've ever seen Dry Zs that weren't potted, as evidenced by the wax around the baseplate screws in all the pics. Here's a quick google image search, every Dry Z that popped up is potted:
http://i.ebayimg.com/13/!BVeK-,g!mk~$(KGrHgoOKjsEjlLmUU31BKSopDWhng~~_1.JPG
http://img522.imageshack.us/i/egf1200sr10bl6.jpg/
http://www.japanvintageguitars.com/images/large/Greco%2059-100.jpg
http://www.plankspankers.co.uk/Greco%20images/SA-1200%20Bld/Dry%20Z.jpg
http://i.ebayimg.com/13/!BVeK-,g!mk~$(KGrHgoOKjsEjlLmUU31BKSopDWhng~~_1.JPG
http://img522.imageshack.us/i/egf1200sr10bl6.jpg/
http://www.japanvintageguitars.com/images/large/Greco%2059-100.jpg
http://www.plankspankers.co.uk/Greco%20images/SA-1200%20Bld/Dry%20Z.jpg