By the mid 70s, all bets are off. It could have been missed by somebody at the factory(thousands were made) or somebody could have refinished the bow during the past 40 years. I read that your bow was marked 45X and you weighed it at 50, so it should have a 50 printed under the strike plate.
My first bow was a 73 Super Grizzly marked 40X and 45, so the question of calibration raised by Richard shows a 5 pound leeway on both of our bows.
I just bought another red striped Super Grizzly from that era, and it is a 55 55. My guess is that the scales were good enough for government work, and not under the scrutiny of USDA inspectors. They were probably too busy keeping the butcher's thumb off the scales at the Grayling A&P!