node "read more" link - in which php file it is

In which php file on Drupal 6 site is the code responsible for displaying the "read more" link for story nodes?

Background:
Approximately 3 years ago I created a drupal site. Because the site was very small, I didn't want to have "read more" link in each story on the main topic page, but rather get rid of this link and always have full story displayed. But I could not find any tool (I know today there is such a module named "read more link").

So I somehow changed the original php file(s) to get rid of the link completely (yes it was a bad idea).

The problem is that today the site isn't small anymore and the topic pages are loading slowly. As I am not a php programmer (I am more in .NET technology), I have completely no idea what was changed in that php file(s) and what file was it, too bad I didn't make any documentation about it. So I'd like to revert back to the original "read more" link functionality.