That seems to be true only of certain URLs. For instance, if it's all one long, encoded string follow by _a or _s then changing a or s to n produces the full-si

That seems to be true only of certain URLs. For instance, if it's all one long, encoded string follow by _a or _s then changing a or s to n produces the full-size image. However some thumbnails aren't formatted in this way and as far as I know can't be (easily) resized to full. Someone please correct me if I'm wrong. Edit: it appears this same char is available in both URI formats I was referring to, so yes, full-size images are exposed.