You could filter the portal for the client to show only the records that match the period you were trying to "find". On the other hand, it would be good to know more about what you're trying to accomplish.
You could add a Filter to your portal:
related::Period = FindField
I agree. Don't "Find", just "Filter"!
I have modified your file to make this work.
I added a KP (Key Primary) field to the Primary (Parent) table
I created a child table that contains the Issue Date, Total and Period.
I added a KF (Key Foreign) field to the child table.
I went to the Relationship Graph and made a link between these two fields and set the relationship to allow creation and deletion of related records in the child table.
I assigned serial numbers to the KP field in the parent table.
I imported the data in two passes to the child table for the Issue Date, Total and Period.
I then created a tab object with 2 tabs and 2 portals on the main screen that both pointed to the child table.
I set the object names up on each of the 2 tabs so that I could use Go To Object  in the Find script
One of the portals I set up with a filter so that it would only display the data in the portal if it matched the data stored in the FIND field.
I modified the find script to work with the new child table and made sure that it redirected the user to the correct tab to show the filtered portal instead of showing all of the related records.
I added a reset button for the Find and a script to handle the reset.
I changed the tables that the other layouts were linked with from the parent table to the child table and fixed the fields.
This should give you what you are looking for.
Wow thats great. As others have 'filter' is the way forward, I didnt know that