I’m missing Purchase order custom fields.
However is seems some fields are taken from Invoices custom fields
In html template I can access it via
$purchase_order.custom2 and $purchase_order.custom3
$purchase_order.custom2_label and $purchase_order.custom3_label
Also the date format for $purchase_order.custom3 is YYYY-MM-DD instead of DD_MM_YYYY as selected in Settings/Localization.