What's an anko number?

I see all these pictures with the word ANKO and a number, I was wondering wha that was. Like I saw a reimu trying to drink a soda, trying to get her drank on, at said Anko 5775 I think

http://yukkuri.shii.org/wiki/index?page=1&query=anko

Behold!

They're yukkuri stories written by the Japanese fanbase and collected in a database... somewhere. And they're given a number based on their order of entry.

Anko also means red bean paste, so it's kind of a pun in itself.