Welcome to the lost world of one-piece forged stems. Quality stems of the ea were forged from a single piece of aluminum, bored for the handle bar, then the clamp ears were drilled and tapped. Most Italian makers were happy to have the ears underneath with the bolt horizontal including Cinelli for many years. But Cinelli was probably the most fashion forward bike maker in Italy, so after a while they went that design one better with this one, which made for a cleaner look.

The thing about these or any forged stem is that removing bars is a matter of finesse, and not using strength or pry bars to spread the stem open. If you look you'll see that the sides are narrowed on the bottom. This allows just enough room to clear the inside of every curve on the bars. You have to rotate the bar as you thread it through to keep the inside where the clearance.

BITD I'd see so many nice alloy bars all scratched and gouged where some oaf who didn't get it tried to force them through rather than use the finesse called for.

There are decent photos of this stem and other nice stems of the era, easily found if you search "forged quill stem images". Obviously, they're random photos, but if you think about it a bit, you can trace the evolution of this design through the decades.