我们于12月9日持广州移民签证（E11和E14)从纽约入境，当时在入境处按了右手食指印，按一般规律应该1个多月能收到正式卡片，可是昨天又收到一封信（跟前天一样的内容，就是notice day 为29日），内容如下：
Notice Type: Welcome Notice
receipt number SRC-**-**-****
case type OS155A immigrant visa and alien registration
Applicant A*** *** *** NAME
notice day Dec. 23,2009
welcome to U.S. We have received your immigrant visa packet from the port of entry where you entered U.S. At the top of this notice you will see a very important number (A-number).This account number is very important to you.you will need it whenever you contact us. The receipt number shown above is simply a tracking number for your immigrant visa packet.
you should hear from us within 30 days from the day shown on this notce....
Note if you have submitted a SB1 visa, you will not receive a new I-551 card. Your previous A-number and the corresponding I-551 card are still valid unless the card expired,......
immigrant & naturalization service TEXAS SERVICE CENTER
我在网上查我的case status 显示为：
Post-Decision Activity
On January 3, 2010, we mailed you a notice that we have approved this OS155A IMMIGRANT VISA AND ALIEN REGISTRATION. Please follow any instructions on the notice. If you move before you receive the notice, call customer service at 1-800-375-5283.
问题是我准备1月9日回中国，在这之前不一定能收到他的信啊，真不知道该咋办？
请问什么是OS155A呀？会让我们做什么呢？会不会需要人呆在美国境内做些什么呢？
多谢！