Close but, 18th century speaking there was "porter" and there was the stronger version "stout porter" as stout just meant strong in those days. Stout appears to have become applied only to strong dark beer in the early 19th C. Martyn Cornell quotes an 18th century gravity for porter as 1071 and mid 19th century gravities as Porter 1050, Stout 1055-1072.
In the late 20th century Guinness Stout used to be 4.2%, their Porter (or Plain) was weaker; OG 1036 at the end of production in 1973 according to Cornell.
As with most beery things it all depends on "when" see also IPA and MILD.
Local ones down here that I can think of without checking are Portobello 4.4%, Wild Card 4.7%, Sambrooks 4.9% and Redemption 5.1%. The Brodies one is 7.8% but they are a law unto themselves and it is described as "Superior".
Old Boots final sentence probably sums it up
You could argue about these things until you're blue in the face!As with most beery things it all depends on "when" see also IPA and MILD.