after 1961 or 1962, Alumacraft created the ALPEX division (just changed brands) to brand their fiberglass boats. the original Alumacraft marquis for fiberglass was a somewhat experimental testing of the waters with "Alumaglass" in 1958 when they contracted for Pipestone to build a few glass boats for them. Then no fiberglass that i know of in '59 then in 1960 Alumacraft had a full line of fiberglass- "Alumaglass" was deemed too confusing so everything was called "Alumacraft" for 1960 and 62, possibly 63, then it was deemed that "Alumacraft" branded fiberglass boats also made no sense since it implied they were aluminum.
So you're really correct that this Alpex boat is Alumacraft- the companies are one and the same and Alumacraft was the original parent company. NEAT boat- did not realize they made some big cabins. I have a 1961 Alumacraft Tangier 16 footer and a 1958 Alumaglass 15 footer- forget the name of the model but probably "Merrie Maid"