Hi!
Unfortunately, the ThinkPad 600 series was introduced in 1998, approximately two years after Lexmark exited keyboard production.
However, to my knowledge, ThinkPads 365C, 500 (although not labelled as such), 750C, 755CX and 850 could come Lexmark's (Model M6-1/buckling sleeves switches, as they're known) keyboard assemblies. I actually recently released an article on the similar Model M4 buckling sleeves keyboards that thanks to assistance from laptop.pics included a part about this type of ThinkPad keyboard - it confirms several of these machines as having Lexmark-made Model M6-1 keyboards which
you can see here (scroll down a bit from the linked section and you'll see some photos of keyboard assembly 'birth certificates' confirming the OEM). But, I cannot guarantee they
always come with Lexmark-made assemblies since Key Tronic picked up production of Model M6-1 keyboard assemblies after "Lexit" in April 1996. The Model M6-1s KT produced are however faithful to Lexmark's design (unlike the 701C/701CS keyboard and its version of buckling sleeves). So if you don't mind a KT assembly, you can add 365E/X/XD and 860 to the list of possible ThinkPads to look out for too.
It's possible there are many other ThinkPads with Lexmark-made M6-1 assemblies too, but there's no way to tell for sure without tearing down the machine. I hope to one day do an audit confirming as many vintage ThinkPad keyboard OEMs as possible (for a future Model M6-1 exclusive follow up to my M4 article), but until then, the best you can try is asking sellers to pop off a keycap and confirm the existence of this specific type of buckling sleeve:
- shark_365X-XD_switches.jpg (725.34 KiB) Viewed 4976 times
Knowing this, you basically have a coin toss between it being a Lexmark or Key Tronic produced assembly unless you know for certain the machine was produced before 1996. And that's assuming there are no other nuances too. But this should work in most cases.
Good luck in your search!
