Chas, you're right. The tapestry reveals a lot about the long putter saga. Here Harold, Earl of Wessex, and his foursome have just checked in with the starter. Harold is picking which cart to use, but one can see two of the members of his foursome on the right have what are clearly long putters:
And here is Harold and his group celebrating at the 19th hole after Harold made a 50 footer for birdie on 18 to win his Nassau:
In this next image, one can clearly see the Odyssey head shape on the long putter above Harold's head (the guy on the far right). This scene depicts Harold being slain by a member of the R&A (the guy on horseback) for using a long putter. Rules making was a bit harsh back then. BTW, the R&A were simply called the "R" back then since they weren't yet ancient.
So, Chas is correct - there is positive proof that the long putter has been used since before the Norman conquest. Even then, the R&A, er I mean the R, were unable to stem the tide of long putters on the golf landscape. Here we are again, 970 years later, and they're still trying.