Is there a way to make the manufacturer code show up on the invoice as well as the packing slip? I see that there is code in the admin_invoice_html.html file for the manufacturer code but there doesn't seem to be a way to turn this on.
 
What table in the database stores these type of on/off settings?

Under Product Settings, scroll down to Appearance, and look for "Show manufacturer code" and check the box for invoice.
 
I don't know if that affects the packing slip though, because we aren't using manufacturer codes.