[mikejmoffitt]

No, this isn't correct. The image isn't "progressive" or "interlaced" until the output stage. Whether or not they are identical is secondary to the timings, which is all that matters. If every frame has an integer number of scanlines (and VSYNC begins during H-blank), it's going to be a progressive scan image.

A "CRT from the time" - the things littered around my apartment that occupy a little too much of my time - can easily illustrate the effect I am describing of a progressive scan image without alternating scanline positions. Go grab almost any '90s game console, or have fun with the Sega Genesis, which supports 240p as well as 480i; Sonic 2 uses 480i to fit two viewports into the two-player mode, but uses 240p during a single-player game.