When Shimano first introduced their cartridge bottom brackets, they made one core size and put different ends on for 68 or 73 bottom bracket shells. Parts suppliers like Quality listed the end pieces as repair parts, so a mechanic could theoretically turn one size into the other. But just as we planned to stock only one size run of complete BBs and a handful of adapters, Shimano noticed what they'd done. They built the drive-side threads into the body of the BB cartridge.
Once in a while we can play our old game with some old stock, but we have to scavenge.
Fight the corporate power, man.