The only "bubble" pieces I've ever seen are those pictured in the book, so Northwood may not have put it into production. The Dugan materials suggest that it was made at Ellwood City, Northwood's short-lived location right after Martins Ferry, where Rubina in Royal Ivy, Royal Oak, etc. was all the rage for several years. My recollection is that the pieces were very heavy and had polished pontil marks. They were owned by a Dugan family member.