Inventory Templates- Help!!

Inventory Templates- Help!!

Im am trying to edit my Invoice template.

Under the billing and shipping details there is a section containing

Account Name:
Status:
Due Date
Sales Order:


All Field are fine except for sales order.
I created an Auto Number for Sales Order Numbers & in Fields list for Sales ORders I have removed Zoho's Sales Order Number (its way too long & not feasible for us to use for tracking)
In the invoice template the existing tag to bring in the Sales Order number was ${Invoices.Sales Order}
This tage brings in Zoho's default sales order number (the long one) but since we use our own sales order number this is useless to us.
We would like to bring our own sales order number into this field. I've tried a variety of tags
i.e. ${SalesOrders.Sales Order Numbers} (exactly the same tag used in our Sales Order Template)
Is it not a possibility for us to bring our own auto number into the invoice template?

Thanks