Hi Kris,
I've seen it several times that the field grey fabric used for the bottom piping is not the same as that used for the top, and I think they aged a little bit different.
A nice officer's field cap, not sure why you call it "summer" and "M17" but that's probably due to my lack of knowledge. As far as I know the regulations changed to field grey peaks in the end of 1915, but I know officer's didn't always follow that.
Regards, Lars