Every costume category in the 501st Legion have different levels of accuracy. Stormtrooper costumes have three: Basic approval, Expert Infantry and Centurion.
The First Imperial Stormtrooper Detachment finally got to approve the 500th Centurion costume a couple of months ago after doing a campaign the past year.
To mark the occation, every eligible* troopers could get one of these pins:
My pin arrived a few weeks ago. I got approved at the Centurion level on November 22, 2017, and I was Centurion number 358.
If you want to learn the difference between each levels, see the CRL: https://databank.501st.com/databank/Costuming:TK_anh_stunt
*) Have one of the first 500 approved Centurion costumes.