Notice the listings beneath where it says Sa Chalerm Thai Circuit Proudly Presents. Presumably, Sa Chalerm Thai was the predecessor to Pyramid (now Apex), which is listed in the 1980 paper. At this point I'm not sure whether Sa Chalerm Thai (Apex) was just the film distribution company or whether it also owned the theaters listed under it.
I especially like the ad for "Nixon's Trip To China," playing at the Century, Srirajwong, Metro and Krungkasem theaters.
In the 1980 listing, some of the theaters that were clearly under different ownership are listed under Pyramid Entertainment Corp.. Perhaps Pyramid (Apex) bought them up, consolidating their ownership over Bangkok's first-class stand-alones. I know that the Hawaii, the New York and the Asia were not originally Pyramid theaters. Then again, being listed under a certain company may also indicate distribution, not ownership.
If anybody has any deeper insights into these matters please feel free to write in.