I've noticed this apparent difference too, but don't have a way of measuring it.
My own Mk 1 was bought in the first week of July 1980 – I can date it because a relative had just died and I got it to play a couple of tunes in his memory. I don't know the history of the other Mk 1, which I bought from an eBay seller.
The windway height on mine appears to be fractionally lower than the eBay whistle. However, it's possible my eyes are being misled by the difference in colour. The 1980 one was very sharp, and dunking the head in hot water to melt the glue made the colour of the plastic lighter. The tuning on the eBay one is pretty much spot on, so I haven't needed to move the head.
I also think the eBay whistle is fractionally louder. They both have the lovely rounded tone of old Feadógs, but the eBay one seems a bit more robust.
All a bit subjective though.

They're both lovely whistles.