With a five-inch-plus head, the “KL Model” has the longest head of any Mills putter.  Given its slender neck, it looks really long in person!  The “KL” designation acknowledges Hugh Kirkaldy and John Low for their role in helping Mills produce his initial KL aluminum putter. According to John Low:

“At Mr. Hilton's request, I had a putter made for him which was supposed to be a copy of the somewhat out-of-date tool which I used ....Mr. Hilton's putter did not turn out quite to his mind; still it was good enough to suggest to him that a club of the same shape might be made of metal which would have the advantages of the wooden putter without its disadvantages. We consulted together, and by the aid of Mr. Mills have become the parents of a club which, for a patent club, is, I think, likely to be useful to golfers (Golf Illustrated, 2 Nov. 1900:99).

From the World Golf Hall of Fame's collection, this putter has its original 33” shaft and sheepskin grip.  The WGHOF number “403” is on the sole.  There is some bag wear to the head, but the condition is still better than most. 

See TCA2 v1 p377 for more about the Mills KL putter.

Letter of Authenticity from the World Golf Hall of Fame