Thanks again, Sticky, for taking another look. I'm not sure that road repairs would have been left so visible and the line fits what I remember seeing in 4 or 5 places in '57. I wish I could remember where they were but I don't recall seeing them at my second visit in '59 as we had a car that year we would have done a lap or two. If this is correct then they existed for at least eight years. I feel my recollection is correct as this fits the bill but their purpose remains elusive and mysterious. Cheers and thanks for helping.