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The sultry bestseller "Fifty Shades of Grey" is causing a boom in the sex toy market -- in fact, a company that specializes in vaginal exercise balls says it's sold a record number of hoo-ha doodads in the last month ... all thanks to the book.

A rep for the sex toy company -- called Fun Factory -- just sent a thank you letter to "Fifty Shades" publisher Vintage Books, writing, "In the last month, our Smart Balls sales have gone up 300%."

In case you haven't read the book -- the main character Christian Grey uses a pair of Kegel exercise doohickeys called Ben-Wa balls on his female sex slave (for those with the book, skip to page 362).

The rep writes, "Thanks to the book, we are getting emails and calls from around the world asking about our Smart Balls ... Yesterday, the Fun Factory office received an email from a 74-year-old woman in Dubai who read the book while vacationing in the USA."

Vagina balls ... uniting cultures.