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Looking at the drawing seems to be a bit confusing. It appears that there are some flats on the connector but the hole doesn't use them. The round notch is called out in a wierd way also. If I were you, I'd contact the manufacturer and make sure you had the correct mounting hole dimensions. It looks like you should use the 18mm diameter hole with the 2mm radius notch but it's unclear why they call out the 17mm flats. Call the manufacturer and CYA just in case.
Testing......