I have seen a few different beers in these most recently GORP and the latest anniversary. I may be wrong since i haven't been to the brewery on either of these release days but it may make their life a lot easier having some pre filled growlers ready to go.
The only beers I've seen them do this with is the mot recent one, their SS 2014 Yirgacheffe and their Shipwreck Porter.
They do it with alot of their small batches. I have a Decoy 2013 sitting in my cellar (orange waxed).
I thought the one liter grower with the ship's wheel in gold leaf/ink was only for Shipwreck Porter. - Joe