What's an Acknowledgment Plan Number number?

[12AX7]

A client receives a late notice for their 2007 5500. One of the options in the notice is to complete Section 1 of the notice if the plan sponsor has already filed the return. In Section 1, the plan sponsor would indicate their Name, Address, EIN, Plan Year Ending and the last question asks, "Date filed with EBSA and Acknowledgment Plan Number number". Can someone please tell me what an Acknowledgment Plan Number number is? Also, the instructions do not request a copy of the return. Would anyone suggest sending a copy with the response. Thanks.

[K2retire]

The acknowledgement number only applies if it was filed electronically using EFAST.

[12AX7]

Thank you.