Is this plaster over gypsum wallboard?
What you see there is a hybrid gypsum/plaster from the 1950s. It represents a transitional stage between traditional wood lath and plaster to modern drywall techniques. You'll probably find metal lath at inside corners and metal corner bead at outside corners and door openings. The gypsum panels are 18 or 24" high. 

I owned a home with exactly that in it for 17 years and consider it to be the best wall treatment ever devised due to its strength and durability, along with the fact that it can be easily cut to, say, add an electrical outlet.