IBM PC 5150 Model Numbers

A comment there confirms my thought when I looked at the numbers:

the IBM PC division kept up that numbering scheme (4-character “type” code, followed by 3-character “model” code) all the way up to when they were sold off to Lenovo in the mid-‘aughts. (It’s also possible that Lenovo has continued it to this day; I just haven’t kept up with Lenovo in recent years, so I’m not sure.)

...and yes, Lenovo continued with it until around 2012:

https://psref.lenovo.com/syspool/Sys/PDF/withdrawnbook/ltwbo...

They switched to longer part number codes after that:

https://psref.lenovo.com/Product/ThinkPad/ThinkPad_P16_Gen_2