IMHO, the delivery note running number should also have prefix.
I don’t really want to add more fields to invoice template, it’s getting too big and bunch of stuff is not even related to invoices anymore.
So the solution is to break the invoice template into invoice/quote/order etc. templates. Once I do this, I will add necessary fields to order/quote template to control prefixes.
Hopefully I will get around to do this next week. Will see.