Advanced PDF - Missing field?

Hi, I am trying to put together an advanced PDF template for Bill Payments (or payment vouchers as NS names them).

Their standard form has the following fields in the section where the bills that the payment is being applied to are shown;

${apply.applydate} ${apply.type} ${apply.total} ${apply.due} ${apply.disc} ${apply.amount}

There seems to have been a fairly major oversight in that the Bill number is not shown.

I have tried every derivative along the lines of;

${apply.id}
${apply.tranid}
${apply.number}
${apply.name}
${apply.ref}
${apply.refno}

All to no avail... can someone please tell me what the correct field name is for that bill number on these new forms.