Assignment Number on Finance Document

When a system upgrade is done from an older version to 4.7, Billing

Accounting Document does not have the assignment field populated with the Invoice Number. To fix this issue a small piece of

configuration is to be done, which is as follows:
T.Code OB16 -Change View " Rules for Allocation Number "
Sort Key 000

- Allocation Number
CHANGE/DELETE THE DESCRIPTION & Save the changes
Once Agian input the Description as it was

prior to the earlier change & Save

What happened here is the system created an internal Code to get the Assignment

Field populated with the Invoice Number.