Last weekend we found these amazing mugs.  They’re very large for vintage mugs (4 inches tall and 4 inches in diameter) and they have a great, heavy feel in the hand.  Thanks to @full_upright on Twitter, who let us know that the mugs were made by Iron Mountain Stoneware.  These mugs were designed by the founder of IMS, Nancy Patterson.  (Patterson’s “NP” mark is under the typical Iron Mountain Stoneware stamp.)  Each of the mugs was decorated by Sally Patterson (Nancy’s sister) and is hand-signed by Sally on the bottom.  We’ve listed these mugs on eBay — if you’re interested, the listings close on 5-12-2013.
It was so much fun to research Iron Mountain Stoneware and to be introduced to the Patterson’s work!  We found this blog to be very helpful as we worked on this post — go visit it to find out more information about IMS.)
Edited to add: This 1975 Reading Eagle article on Nancy Patterson gives some information on the connection between IMS and the Royal Copenhagen company.  It seems that Royal Copenhagen produced Iron Mountain Stoneware for national distribution, which clears up a mystery of @Luolashop’s, where Iron Mountain Stoneware has Royal Copenhagen stickers.