I am pretty sure it is legal. check out usga groove rules
http://www.usga.org/InfoClubsDB/index.asp and put in Acushnet for the manufacturer and Titleist for the product name.
Cathi @TeamTitleist 1-888-TITLEIST
Are the 690.mb irons legal to play also. What is the difference between the 690mb and the 690.mb?