Looking on Brigade QM's web site there are two styles of poncho liners... a 'GI Type Poncho Liner', and a 'GI Issue Poncho Liner'. First one being 25 bucks, and the second is 45 bucks. Only difference I can see is that the more expensive one is a bit bigger.

How small can you pack these?
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"One should not increase, beyond what is necessary, the number of entities required to explain anything"
William of Ockham (1285-1349)