20-amp only? That must be reserved for some specific bit of hardware, best I could guess. That would make more sense in a hospital. I've never seen a 20-amp only outlet, always the typical 15/20 that has the T on the neutral for "compatibility".
One thing I do these days with new venues is I have one of those pen-type devices that can detect a live circuit. If I have a live circuit, next thing I do is meter it and document that on a per outlet basis. I've only run into 1 problem: an outlet was only providing 97 volts so I had to pull from another circuit. I told the owner of this problem and they got it fixed. The rest of the time, it's the typical "we show 120 volts" but when you apply a load, the voltage drops due to weak power entering the service panel.
Yeah, I can see a 20-amp ONLY being a PITA. If I were you, I'd just make a quick pigtail that adapts a 20-amp Edison male to a 15-amp Edison female and be done with it. I'm sure you've got a cable you can sacrifice, or at least lop off a bit of. I bet in your case, you've got some old but decent cables lying around that could be recycled in part for this project.