Our AI overlords say that GM mounted it upside down to keep water away from the high tension terminal. Perhaps 75+ years ago without the benefit of silicone rubber, this may have been valid.
However, I've seen many different antique vehicles with the coil mounted straight up, sideways, etc. My 1939 Packard mounts straight up. 1935 Auburn mounts straight up , as well. If there was a real danger of water getting in, I'd think the luxury makes would have mounted it downward or installed a cover or something. They didn't so I think that reason is bogus.
AI also says that mounting it upside down means the wire could be made shorter, costing less money. Seeing some of GM's other cost cutting measures, that sounds like the primary reason to me.
