eiru_extra_purchase/views/purchase_order_view.xml

<?xml version="1.0" encoding="utf-8"?>

<openerp>
    <data>
      <record id="purchase_extra_view_order_form" model="ir.ui.view">
          <field name="name">purchase.order.extra.form</field>
          <field name="model">purchase.order</field>
          <field name="inherit_id" ref="purchase.purchase_order_form"/>
          <field name="arch" type="xml">
              <field name="partner_id" position="after">
                  <field name="referencia_venta"/>
              </field>
          </field>
      </record>
    </data>
</openerp>