I think labrax is right on. Light amber could be one of the original S-glass blanks dating back to the early 80s, or it could be an even earlier E-glass "Cutter" or "Supercutter" blank. In any event, it's definitely an 11-6 cut down. Being as stiff as you describe it, I would tend to guess that it's an early 1165 or 1166 blank cut down to 10'.
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