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I'm not sure if this is the specific issue, but there are two similar but not quite identical thread sizes that you find on the backs of various small SCTs (depending on make and age).
The "standard" SCT thread is 1.375" * 24 TPI (1.375" = 34.925 mm; TPI = Threads per Inch). The original Celestron C90 also has these threads on the rear port, and I think this is also the thread size used on the larger Meade SCTs.
The Meade ETX90 rear port is slightly different, but close at M35-1 (i.e. 35 mm diameter, 1.0 mm pitch).
ETX accessories (with the slightly larger metric female thread) will thread on to the slightly smaller C90 male thread, but bind after a few turns. You generally can't get the slightly smaller "standard" accessories onto the Meade ETX metric male thread.
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