Relating transaction lines between purchase order and sales order

First, I apologize if this is covered somewhere already. I searched for a couple hours in various places and looked at the Records Browser, but could not find an answer.

Is there an transaction line field that relates lines between a purchase order and the sales order from which it was created. Take for instance:

Sales Order 1

1. Item A
2. Item B
3. Item A
4. Item B
5. Item C

Purchase Order 1

1. Item A (from line 3)
2. Item B (from line 4)

Purchase Order 2

1. Item A (from line 1)
2. Item B (from line 2)

Purchase Order 3

1. Item C (from line 5)

We need to know which line on the sales order each line on the purchase order is related to. Since an item can exist on an order multiple times, using the item name isn't possible. The line number fields on the transactions are specific to that transaction.