subtitlecat page ID differs from download ID

On subtitlecat.com, the numeric ID in the page URL (e.g. /subs/570/foo.html) is NOT the same as the ID in the actual SRT download URL (e.g. /subs/573/foo-en.srt). Guessing the download URL from the page URL fails with 404. You have to fetch the HTML page and extract the real download link. IDs also do not increment predictably per episode — adjacent episodes can share or skip IDs.