Sales Orders to Purchase Orders don't have a LinkedTxn table.

Sales Order Created in QuickBooks

     Created the following Sales Order in QuickBooks:

Purchase Order Created in QuickBooks

     Get QuickBooks to create a purchase order. It made the following:

With the "Sale Order 4:" link in the memo line.

Find Purchase Orders for a Sales Order

     This means I can find the purchase orders for a sales order by doing the following:

     SELECT RefNumber, VendorRefFullname, PurchaseOrderLineDesc,
     PurchaseOrderLineQuantity FROM PurchaseOrderLine
     where Memos like 'Sales Order 4:%.'

Write the PO# Back into the SalesOrder

      However, the best solution would be to write the PO #back into the SalesOrder. This can be done by doing the following:

      UPDATE SalesOrder SET PONumber='42' where TxnID='5BDD-1197759656'

Result in QuickBooks