Here is confirmation of what some here may have already known but of which I was unconvinced:
From: DRT To: C. da Silva Subject: Dalva and Presedential Vintage Port
I am compiling a database containing a list of all vintage port declarations from each port shipper. I am aware that C. da Silva produce vintage port under two brand names: Dalva and Presidential. Can you please tell me whether or not these are in fact the same wine sold as two different brands or are they two completely different wines?