Thanks Michael. As these are when we first got going in 2014 then the file numbers are small and we’ll keep them where they are. 
Interesting enough, we did move server recently, and went for a Discourse backup / restore route (rather than upgrade in place of the base Unix version) and I think (although not 100% sure) that the restore did not put these local files in place properly. They were contained in the backup archive but the restore process only seemed to work for optimized/originals down.
It wasn’t a big deal, because we could tar -x them ourselves from the backup archive (when we noticed the old and new servers were different in their uploads/ contents), but something that might trip someone up, so wanted to mention here.
Even though 99.9% of our uploads are served from S3 now (we switched from local to S3 fairly early on) I think that we must have copied up the local files when we manually create the S3 bucket initially. On retrospect we probably should have rebaked posts, but it always worked well enough with the very small (and old) posts have the local file upload URL).