I used the Apex Tactical 100-063 hard sear which I purchased from MidwayUSA here. This sear is for the S&W M&P 45 ACP and M&P Shield. There is also a video on how to install the sear at MidwayUSA WEBs site. Just click the video tab at the web site or watch it on Youtube. The video also shows you how to test the sear fit in the pistol. When I tested my reassembled pistol I could also here the striker spring unload and the trigger reset when I released the trigger.

"The Apex Hard Sear was designed to enhance the performance of an already outstanding pistol. The modified geometry will not only improve the over-travel and sear reset characteristics, but will also dramatically reduce the trigger pull weight in most pistols. This sear is made from heat treated A2 tool steel for durability and long service life.
Note: Works with all current production Smith and Wesson M&P 45 series pistols, as well as the M&P Shield. NOT currently for use in the Shield 45. Does not fit in M&P series pistols chambered in 9mm, .357SIG, and .40S&W – please the M&P Fully Machined Sear."

The date code for my Shield 9mm is 02-Feb-2015.