Joe - You need to remember that 1959 was years before AMO and standard lengths. Lots of those old Bears varied in the actual length of the limb and the position that the string groves were cut into the limbs. The people stringing and marking the bows, just put the closest string that would work on bows like yours and assumed it was a 64" bow. Those odd marked lengths show up once in a while.
But if the bow wasn't build on a different form, it really isn't a different bow.
All Bear bows of different lengths were built on different forms.
I have a couple of Bear bows that are lengths that are not shown in the Bear catalogs, but these bows were actaully built on different forms with different riser lengths and are legitimate bows.
Hope I haven't confused you.
Is your 64" Grizzly the same one we talked about last August? FG875A, 49# ?