A storage room full of vertical files.
Vertical files full of photomechanical transfers, paste up boards, sizes and sizes of photostats, type sheets. These took up rooms, often warehouses to store if the agency was a bigger agency.
Then Pantone chip definitions and swatches. This is were Pantone was born and blew up. A color system which was consistent simply by speaking a number over the telephone. Or writing a number down on a mechanical board. It was honestly revolutionary at the time (before my time, but I still appreciate it).
If something was to be printed, you sent, via postal mail (when it was still reliable) or FedEx, photomechanical negatives and specifications detailing trim size plates, Pantone colors, or processes breakout percentages. You actually sent a black and white photomechanical negative and had to specify 20C 10M 40Y 30K for a logo.
Before the digital revolution everything was done by hand on paste up boards, then shot with a copy camera and the negatives stripped into plates. It's was very common to store the paste up boards and the negatives for each and every projects. Sometimes plates would be stored but often they were unnecessary if you had the negatives.
For even the simplest print projects you would have a minimum of 3 to 4 physical pieces to store.
Clients generally had either a printed piece or a photostat. Things were actually a bit easier at times because the client was aware "these files are for printing". Therefore they treated them with extra care and passed them along when asking for new projects. Clients understood you could not just find an advertisement and rip off the logo part and give that to the designer.
Markets were also very, very, local. Clients didn't hop and jump from designer to designer because of all the assets the current designer had. It was a major undertaking in some cases to collect all print-related mechanicals and deliver them to a client. There were also often substantial costs involved, which was better understood by clients. There were actual physical objects so charging for delivery of production files was understood. Clients had a tendency to stay put. Unlike today where clients just see a digital file as any digital file and don't associate a value with any of them because they assume they are the same as the digital files they themselves can create on their computer.
There was far more of an art to print design. You had to have craftsman skills to create mechanicals properly. If you couldn't draw a straight line with a ruling pen, you were weeded out fairly quickly. Competition was much, much, less prevalent than it is now. Since the market was so local, there often weren't a great deal of choices unless you were working in a large metropolitan city.
Brand consistency was maintained by sticking with the same agency/designer. Which is still true today. The problem today is that there are so many more people in the field and access to the tools has become easy. There's little way to differentiate those who understand what they are doing from those that simply own the tools until you work with them on a couple projects. Clients can get sidetracked by cut-rate pricing and misguided proliferation of the idea that anyone with a computer and Photoshop can do what needs to be done.