Good observation, Rick, but what you gotta keep in mind is that nowadays a lot of substitute materials are coming in from overseas and it's hard to know exactly what's in them sometimes. Even my dedicated plywood supplier couldn't give me a precise answer yesterday on an analogous question (some of these questions are code dictated locally). The mere thought of masonite etc would probably stop the pacemaker
of any real art conservator's heart. It might be fine for casual craftsy use, or temporary commercial displays, but certainly doesn't belong around anything of serious quality or intended permanence. It ain't all that flat either, which will be apparent if you try to mount a glossy
print to it. Ordinary fomecore isn't very smooth either. Sintra is so-so. Dibond... well, ya get what ya pay for, and you'll pay for that!