Hi Richard, thank you for another helpful write up.
It is a while since I have pulled a Nikkormat apart, but I recall that it has an internal black spongey seal just under the penta prism, which can drop pieces of decaying gunk straight down onto the shutter assembly. Taking the top off a camera body increases the amount of effort quite considerably, but sometimes there is little alternative if there are internal seals that have decayed.
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I have quite a soft spot for Nikkormats - I probably have more of them than some folks have 55mm Micro-Nikkors.
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The black 1977 vintage FT3 is my favourite and was my second SLR and I purchased it as New Old Stock in 1982, although my first SLR was a chrome 1974 FTn. I purchased both of them new and I still have them in running condition. And I still use the FTn's 50mm f/1.4 SC Nikkor on my digital bodies - it now has a factory Ai aperture ring and one of Bjørn's metering chips. Still works flawlessly.