今天收到移民局的Welcome Notice

今天收到移民局的Welcome Notice,給了一個SRC的號碼，我上網查了以下USCIS的網站，發現：

Receipt Number: SRC05251521**
Application Type: OS155A, IMMIGRANT VISA AND ALIEN REGISTRATION
Current Status:
This case has been approved. On September 23, 2005, an approval notice was mailed. If 14 days have passed and you have not received this notice, you may wish to verify or update your address. To update your address, please call the National Customer Service Center at (800) 375-5283.

請問是否綠卡已經寄出了？為什麼不是Green Card ，而是Notice啊？？