5-speed manual

[seo]

Former Mercedshop forum member here, looking for some input.
I'm preparing to switch the Otto-matic out of my 1985 300 turbo, and swap for a manual out of a retired euro-market 300d NA. The manual is a 5 speed, the plate says "Teil nr. 123-260-2001 aggregate /80 024 716. This transmission has 200k miles on it, but works fine. It does have an odd fault. Engine running, in neutral, with clutch engaged, the transmission makes a "clanking" noise, sort of like the sound that a bad throw-out bearing would make when the clutch was disengaged. I've been told that this is a problem with a layshaft (for reverse gear?), the layshaft runs in bushings in the case, and get end play and make the noise. That's what I've been told, anyway. Some people say it's not a problem, others that it will cause trans failure, either way it's not a nice noise.
So, does anyone know about this problem? Is it easy to fix? I've heard about re-bushing, or boring a larger hole and installing a ball bearing. Do you know of anyone who works on these, or where parts are available. I'm in Maine.
I've been told that this is the same transmission as was found in some 280E gasoline cars of the same age. Was it ever imported to the US by Mercedes, and do they have parts for it? Lots of questions.
Thanks very much in advance for any feedback or advice.

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