Hi Jon,
There’s no built-in variable named “stimulus_presented”. That must be defined by the particular experiment(s) you’re looking at, so I can’t say when and how it’s being set.
The usual way to get info about stimulus presentations is to extract the variable #stimDisplayUpdate from the event file. The value of that variable is a list of dictionaries, where each dictionary contains the tag and properties of a stimulus that was on screen during that display update. For image file stimuli, the dictionary contains a filename key, whose value is the full path of the image file loaded by MWorks, and a file_hash key, whose value is the SHA-1 hash of the image file (more info). If the filenames used when running the experiment don’t match those in your database, the file hash should provide a way to create a mapping between the two.
Does that answer your question? Apologies if I misunderstood: This is my first day back at work after three weeks of vacation, and my brain isn’t back up to speed yet 
Cheers,
Chris