Odoo-Build/snippets/purchase.report_purchaseorder_document.get_position.xml

<data inherit_id="purchase.report_purchaseorder_document" priority="50">
    <xpath expr="//table/thead/tr/th[1]" position="before">
        <th>
            <strong>Pos</strong>
        </th>
    </xpath>
    <xpath expr="//table/tbody/t/tr/t[1]/td[1]" position="before">
        <td>
            <span t-esc="line.position"/>
        </td>
    </xpath>
</data>