The Sig Sauer P229 E2 9mm and Sig P229 made from late December 2010 now has a wider magwell which is the same a the Sig Sauer P229 .40/.357.  For those pistols you need to use a Sig Sauer P229-1 magazine. While these will lock in to place they will have some play in the magwell that may cause reliability issues. These magazines are for the P228 or P229 9mm that have the narrower magwell.  

Constructed from certified carbon steel and features a heat treated housing and high-impact polymer or certified carbon steel follower and floorplate. The magazine spring is made of high strength type "D" music wire-the strongest wire available. TIG welding provides additional strength and a seamless finish. Each magazine surface is specially treated to ensure high corrosion resistance.

Mec-Gar is proud to be the Original Equipment Manufacturer of magazines to most of the prominent firearm manufacturers for the last 40 years including AMT - Armscor - Astra - Beretta Bernadelli - FN Browning - BUL - Charles Daly - Colt - CZ - IMI - Kel-Tec - Kimber - Llama - North American Arms - Remington - Sigarms - Smith & Wesson - Springfield Armory - STI - Standard Arms - Star - Steyr - Tanfoglio - Taurus - Walther