Ok, so I got to check my "warehouse" and turns out I have 3 520STM models.
1) UK model with TOS 1.2. 512KB RAM
2) SWE/GER hybrid. SWE keyboard (a bit beat up) and German TOS 1.0 also 512KB RAM
3) German model currently in pieces under work. 2.5MB RAM, TOS 1.4 UK, I believe an RTC device, not sure, also some external switch. This project is on hold.
My external floppy is acting up so I got the info from a test cart.
Keep in mind, I only buy cheap, as is, for parts, to restore. These 3 have not been worked on despite one being in pieces. However, these are resilient little things and seem to power up and load the desktop.
If you're still interested we can arrange something, I'm sure.