The 500-Series was marketed as a model range which was relatively full sized, but without removable drives which take up a lot of the space. Therefore none of the 5xx-Series machines accomodate a built-in magnetic or optical drive. They tend to be larger than the ultraportable 2xx-Series, aimed at the enterprise above the consumer 3xx-Series and yet smaller than the full size 7xx series machine (which often had a magnetic drive, an optical drive, or both!
There wasn't a 520 or 540 Model and the 5xx models aren't strictly chronologial for 2 major reasons. Firstly, the 550/555BJ came out very early as the transition from portable and japan-centric notes emerged - which you can somewhat see in how the 550 doesn't have a trackpad, whereas the 555 does and they're fairly big. I guess these should not have had the 5xx model-range attributed to them.
Then you had stuff being created in at least two different labs, somewhat competing and then merging or taking over. Both were creating 'small' and 'medium' machines, and the US ones started more like small and got bigger.
500 (Lexbook SE10) - USA
This blue-logo'd machine contained a 50 MHz 486SLC2 and a 85 or 170 MB HDD with a Mono Display. Designed in the United States by the same team who went on to design the 701c and 701Cs.
Here we have the author's 500 on the left with the monochrome screen and blue LCD bezel buttons and 510Cs on the right, with a partially repaired vinegar syndrome screen.
510cs - USA
This colour-logo'd machine had a similar in design to the 500, but without the distinctive blue contrast and brightness buttons, a colour DSTN display and a blue lightening processor. The brightness and contrast controls where embedded as softkeys under regular keyboard buttons. It has a weird breakout box which came with the machine which allowed you to connect things like an external video display and a floppy drive via the dock connector.
- WD90C24A2 video controller with 1MB
- 7.7" STN display with 640x480 resolution
530Cs - Yamoto
Often aligned with the smaller 230Cs and share the same manuals. It was designed in Yamoto, Japan, whereas the 500 and 510 came from the US.
Here is a side-by-side comparison of a 535X on the left and the 530Cs on the right (or the 530Cs being on top). You can see that the 530Cs is slightly smaller than the 535 and has a slightly older design logic with a painted metal case, rather than a black plastic one:
535, 535E and 535X - Yamoto
Very nice to work on, but best with a TFT screen, as DTSN was also an option. There is a dodgy hibernation battery inside the earlier models which tend to corrode the inside. I've done a bigger article here. I think it only was released in Asia.
Easy to dismantle but be careful of the hairline pins/traces which can very easily be knocked on the earlier 535 models with a more exposed CPU.
550BJ (Canon)
Came out before the 500 machine, before IBM even knew what they were doing with their model designations/numbering scheme. Sat alongside the 350 (weird PS/note rebadge) and the 720 (a step on from the iconic 700C). The 550BJ is a strange collaboration with IBM Japan and Canon Japan with a built-in bubblejet printer (hence the name!) and a monochrome display and no trackpoint. I've done a full article and teardown on this model here: https://ret.rocks/index.php/ibm/500-series/ibm-thinkpad-550bj
Weirdly the 550 and 555 machines were a complete departure from the rest of the 5xx-Series because they were full size machines, made even bigger by incorporating a printer and floppy disk drive. There were various other Canon-branded machines which share a similar design and ethos to the 550.
The 550 suffers from a wide array of surface mounted elerolytic capacitors that are all going bad now. I had 4 and could only get 2 of them working, because the leaking caps had damaged so many different traces on the motherboard.
555BJ (Canon)
Similar with the 550BJ, but with a trackpoint and a colour screen.
560, 560E, 560X and 560Z
Another design direction more closely aligned with it's bigger brother, the 760, the 560 series was a more widely released small notebook machine. The 560 series feels more plasticky and lightweight than came before, even if the 560Z does tend to cram a lot into a small space and feel a bit weighty again.
The 560Z was very different to the other 560 machines, although it wasn't obvious on first sight. It only came with the largest 12.1" TFT display, the Z came with the fastest 233 or 300Mhz CPUs and the HDD was accessible by a removable door on the outside of the machine.
570 & 570E
Again, more widely-released machine with Intel Pentium 333Mhz to 500Mhz CPUs and either an SVGA or XGA display. Marketed as a 'transformable' computer which was relatively light without any removable drives, but easily docked. It bugs me that the last 'official' western 570-series was an E which would either signify 'cheap' or 'cut down educational market' model, which is isn't. IBM released the 570E and decided to start lettering models. The 570E was replaced (alongside the 240) with the X series.
Yeah, so there was no 520 (because it kinda overlapped with the 230Cs) and there was no 540 because it didn't make sense inbetween the release of the japanese 535 and American 560. The 550BJ came before all of them.