JoomyI have got one from Massdrop in November of 2017. It is a universal leather A5 notebook cover. One notebook insert came with the cover as extras. The cover quality, I mean leather and manufacturing, is absolutely stunning. I am definitely going to get one more.
ArtyKatIt is a universal A5 notebook cover and A5 notebooks are probably the most common ones. If you are asking about the original insert, I have no idea and cannot help here. The main purpose of the cover is to host "any" A5 notebook you like.
ArtyKatSeems like a generic A5, thick enough to take Nanami Seven Seas or Leuchtturm 1917.
Actually, you'll want to stay away from thinner notebooks such as a Midori MD.
JoomyI think I have to express myself better. @ArtyKat is right, there is enough space to host a thicker notebook so if you use a slim one, there will always remain some extra space. On the other hand, the leather is rather thin and soft so it will adapt quite well and definitely better than some other and stiffer covers I had in my hands. It is even finer than it looks on some of the product photos. Moreover, not all "A5" notebooks are sized exactly the same. :)