A check box formatted field could be used to lock each record, but it would not have anything to do with the record number.
Say you define a field named "Lock" in the Order table and format it with a check box such that clicking the check box enters a 1 in the field when it is to be "locked".
Then you can use:
as the expression to use in Manage security to control edit privileges on the order table and
as the expression to use to control edit privileges on LineItems.
Also add this expression as a validation rule in LineItems:
on at least one required field to prevent users from adding new related records to LineItems.