XK was a Pale Ale one strength up from AK. The K tells you that it wasn't a Mild Ale. These were Fuller's Pale Ales in 1887:
AK 1049.6
XK 1057.1
IPA 1060.9
X is a strength indication. The K tells you that it's a Pale Ale. The strengths go A, X, XX, XXX, XXXX.
I've spent years working this...