The point that is often missed is that best work guns usually closely followed the prevailing style of the day. Stocked-to-the-fences is a style that supplanted the flatback style. After STTF became accepted, flatback was relegated to lower original quality grades. Flatback SL's ofbest work OQ made prior to this shift in style are still best work guns. However, an "A" grade SL, flatback of STTF,is not a best work gun; then, now, or ever. So, the presence of a flatback, while not an invariable mark of lower OQ, is a signal to determine the guns OQ before shelling out best work gun money. Same for lump through the floor plate.
