If you download the current version of your document, right-click on the download in the Downloads window of your browser (Firefox) and copy the URL. Then simply append &revision=NNN and paste this new URL into your browser address bar and hit enter. It will download that specific revision.
So the only exercise left is finding out which revision ID you need to use.
In order to do that, visit developers.google.com revisions list and click on Try it now, then enter the fileId of your document, which is the long string of characters and numbers between docs.google.com/document/d/ and the next slash in your browsers URL bar when editing the document. It's also visible in the download URL you copied before. Paste that into the fileId on the API explorer page an hit Authorize and execute.
Scroll down to the response, look at the modifiedTime and choose the correct number from the id field. Then use that number to download your document. The final URL should look something like this:
where XXXX is the fileId and NNN is your revision number.