We had a Fortran programming class in HS, with no computers. The teacher was affiliated with the local university that had one that filled an entire room. Or so he said. He'd take our work there and have them convert it to punch cards. A classmate copied mine because he was so confused. Our two were the only ones that worked. Busted. That would've been around 1973-74.:O)
The first one I ever saw and worked on was at work. The company had one, that's "1". Don't remember the make but it had 2 floppy drives. No GUIs, all line coding. Only a hand full of us got any use out of it. We used it for budgeting projects then eventually at sites to speed up settlements of the project. Rock concerts, actually. We thought it was the best thing since sliced bread. That would've been around the mid '80s or so, I think. ;O)
Slide rules were a computer of sorts. I've seen them described as a mechanical calculator.
Well, of course they are a mechanical calculator, but very limited in scope compared even to the earliest electronic calculators.