I haven't kept an accurate count of the Kennebec models I have seen over the years, but I rather suspect it would total somewhere between 15 and 20. So, I may have seen close to 25%, maybe 33% of what they claim to have made by way of 'display samples'. That really isn't definitive, though, of the number of 'survivors' that are still 'out there'. Time and accidents have a way, usually, of decreasing the numbers. Whatever the actual remaining total may be, it is not large to begin with. To have made only 60 samples over 30 years of manufacturing history is not many to begin with. If I recall correctly, Kennebec made around 30,000 full-size canoes during that period. So, sample production was a very minuscule part of their focus.
As for your sample, I am going to speculate (based on your photos) that it measures 63" in straight line extreme length. I base that guess on the blunt ends of the gunwale caps. All the 63" models I have seen have a rather 'spade' end to their caps; the longer 68" samples all seem to have gunwale caps that end with an angled,bias or pointed cut. The seats with their basket weave caning match all the other original Kennebec samples I have seen. Everything about your models looks to be very original, and is also the nicest one in the best condition. Thanks to you and your family for looking after it so well over the years.