I can't find page.php; nor is there a "core" subdirectory of concrete... nor is there a page.php in Concrete/models...
What am I doing wrong?

Are you sure you're using Version 6 not Version 7?

I found it... under /updates/concrete5.6.3.4/Concrete/Core/Models
There is a Concrete directory in the root along with /updates, so that threw me off.

This fixed the problem! Thanks for your help. I had tried to find the answer myself, but the search in the Forum is almost worthless; it seems to pull up random unrelated stuff. It steams me that a version, known to be inoperative, is left on a file server and lauded as if it worked.

Anyway, that is just me venting. This upgrade process has me pulling out my hair.

I did manage to import most of my content, and other parts were straight html cut and pastes.

Thanks again for your help.

I know what you mean about the forums returning seemingly unrelated results, one thing to check when you're searching is that the "Sort By" on the right side is set to Relevance not Recent Activity. I'm glad this got you going and yes, moving from 5.6 to 5.7 is a huge undertaking, but hopefully it will be worth it in the end.