Hello dieandere,
I know, it's an old thread and maybe you're still inaktive in the forum, but this is just the second TS400 transition model i ever saw "in the wild". So i have to reply lately to confirm it's a '78 TS400.
Mine is the burgundy colored model and nearly in pristine conditions, all stock, except for the number plates on the volume and tone knobs, like yours. The tuners works fine after all those years and holds the tuning very well. Theres no corrosion at all and i like the Protomatics II
The serial number is 058441, so not far from yours. You have a fine guitar there, i hope it's still in your possesion.
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greetz n'roll