I think you'll find that on-board weight and balance gauges go back way before the eighties. They were certainly fitted in BA's 707 freighters back in the seventies when my hair was still brown. They were never particularly accurate, but were good enough for checking the honesty of those who declared the weight of the cargo items. There are plenty of unscrupulous cargo agents in this world, who don't give a damn about the load sheet as long as they get their back-hander from the shipper. Without an on-board system, how do you double check?
Now, you can argue that the declared weights on the load sheets are correct as much as you like, but the worst I've seen was an Argosy that flew into our airport with a loadsheet that showed that the cargo weight was within the aircraft capacity. The captain insisted on having it all weighed in front of him during the unloading process, when it was found that the actual weight was almost double the declared weight shown on the loadsheet.
As it was, at the departure end this aircraft had barely made it off the end of the runway and luckily had a nice obstacle clear harbour to gain a bit of height. Being now fully aware that he was grossly overweight, the skipper elected to continue and burn off all the fuel, rather than take the risk of landing as he was.
An on-board system would certainly have picked up such a massive load sheet error.