3.5” disks were extremely common. Everything from Atari STs to programmable keyboards supported them. Plus they were the standard in PCs for a very long time.

3.5” disks were extremely common. Everything from Atari STs to programmable keyboards supported them. Plus they were the standard in PCs for a very long time. What the GP saw was 3” a disk (ie half an inch smaller). Those came with a harder plastic shell and were a lot less common except for a few specific systems (like the aforementioned Amstrad 8bit micros).