Um.. duh, what was I thinking? Somehow I went from discharge to intake between when I read your post and when I answered.
I remember on Brenda Novakowski's restoration pages that she mentioned getting all new "service" hatches on one of their 1970s Trilliums; I wonder if she found one?
Okay, I looked and I can see she used round ones. Of course that doesn't mean the square aren't available, but it does look as though the round ones covered all traces of the old holes. It shows up about 1/4 of the way down this page:
The rest of her pages are pretty handy for seeing a Trillium restoration in progress, as they've done a few of them. "Dewey" was the most extensive.