The pins where changed to include one or two voids in the 60's I believe. Pins became heavier over time as advancements were made in the manufacturing process. Solid pins originally weighed 3 lb 5oz then in 1947 pins started being laminated out of several pieces of wood. The glue increased the pin weight slightly. By 1954 pins had both a plastic coating and a plastic base which increased the pin weight to 3 lb 8 oz, then in 1961 the neck size was increased which increased the pin weight another oz.
I swear I used to have info on what happened after that but I can't find the info now. The neck size was increased in 1961 and the voids were added after that. If memory serves me this decreased the pin weight to 3 lb 2 oz and that was the lightest pins ever were. I believe pins are actually HEAVIER now than they used to be weighing up to 3 lb 10 oz.
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