Chris,
Just from what I have seen of these Kahn canteens, the covers seem not to fit. It is possible that the papercloth material shrank when exposed to the weather in the field and therefore they made them a bit larger than the canteen to allow for this shrinkage.
The glass bottles were obviously larger than their metal counterparts, so perhaps the covers were made of a size to accommodate either canteen. Regardless, it appears that they have been together since the war, so it is not a case of two similar, but unrelated, items being "married".
Some of these canteens have already shown up on Emig's site for 160 Euro. I think I saw three and all were sold.
Chip