> it wasn’t called a computer then, because the word “computer” back then meant big mainframe systems that ran batch calculations
PDP being a minicomputer (at the time), it could do that too and cheaper. What I heard the reason for this was to circumvent procurement restrictions. The word "computer" meant a high level of scrutiny, but obviously a "Programmable Data Processor" isn't a computer because it doesn't have the word "computer" in it (duh). So just a sales / marketing trick.
To round out the timeline I'd include the TX-0, first computer based on transistors, and essentially the experimental prototype/proof of concept of what became the PDP-1. https://en.wikipedia.org/wiki/TX-0
then on the other end of the timeline, the DECSYSTEM-20 which wasn't any longer called PDP but it was simply an upgraded PDP-10. https://en.wikipedia.org/wiki/DECSYSTEM-20
To round out the timeline I'd include the TX-0, first computer based on transistors, and essentially the experimental prototype/proof of concept of what became the PDP-1. https://en.wikipedia.org/wiki/TX-0
then on the other end of the timeline, the DECSYSTEM-20 which wasn't any longer called PDP but it was simply an upgraded PDP-10. https://en.wikipedia.org/wiki/DECSYSTEM-20