An update, I made the HDD PCB for the Tecra 520 > 550 series laptops and it works on my Tecra 520CDT - see that mess of wires? That's what I was using prior and it keeps breaking:

It's very definitely version 1 - I had a deadline for making it and after declaring it done and comparing it with the original I found that the HDD connector on the thin end was backwards... I had to re-do it and finally put this out at 4am dead tired. It means the compactflash doesn't have a lot of space to work with and probably no other type of drive would work.
I think it can be resolved by using a cable but the dimensional constraints of the Tecra 520/30/40/50 are very limited so only the original flex cable can properly fit with the caddy.