thanks for the info, Josh, and - when I opened that template in the admin editor - I found what seems to be an elision (is that the right word?), as though two sections of code, one for the mark as edit and the other for preview, had been run together, with effectively a whole section missing.
Just to test, I added a line break after the section of code for the post box, because there was a little check box at the SIDE of the big box for the post, rather than under it. My change put the check box under the post box, with the text "I want to preview my modifications and add or edit an attachment."
But there IS no check box or text for selecting whether or not to mark the post as edited, and I think there should be!
What do you think? Tony
EDIT: I've stared at the template a bit more, and I think my remark about the elision is wrong. BUT the fact remains that the check box was to the right at the post box (which my line break fixed), AND there is no mark as edited option when I (as administrator) go to edit a user's post. (Would there be a difference if I went as moderator? For the case I tried, I'm both...) But the mark as edited option IS there if I (as admin) edit one of my own posts!