Well, that's what differences are for. I grew up in the car industry, specifically GM, and the parts of my youth that weren't misspent were well spent in the dealership garages. I've wrenched on quite a few muscle cars as well. I used the Wikipedia article as a reference solely because it appears first in a search for references.
How about Super Chevy Magazine? Would you accept their description of the first big block being the 348/409 W series? Or maybe Peter Sessler's book "Ultimate American V-Eight Engine Data Book, 1948-1974, this excerpt from page 49, Chapter 6, Chevrolet W-Series Engines 348, 409 & 427 (emphasis mine):
Almost at the heels of the successful small-block V-8s, Chevrolet came out with a big-block engine that would develop more low end torque for large passenger cars and trucks. Rather than just make a bigger version of the small-block, Chevrolet decided to experiment, and the result was the W-series big-block with its unusual cylinder head desing. In 1958, Chevrolet released the first version of its new Turbo-Thrust engine, which measured 348 cid.
As the development of the big block progressed, GM retroactively dubbed the "W" series the Mark I. The Mark II was an experimental engine used by Junior Johnson at Daytona in 1963, which qualified fastest, but did not finish. The Mark III never got off the design bench. GM called the Mark IV block "Generation 2", to differentiate from their first attempt with the "W" series. The Mark IV was the first big block to be called a "Rat Motor" as opposed to the small block "Mouse Motor". Want to make a 481 cid engine? Put a crankshaft from a 454 into a 409 block. Go ahead, they bolt right in without any modifications. Indeed, many of the parts from the "W" series were used in the Mark IV engines.