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It means, most likely, that a MicroSet device has been used or can be used on the clock. Information about one type of the device can be found here: http://www.bmumford.com/mset/modelwatch.html The MSet is a device which will keep the clock performing to its peak accuracy. You should be able to bring it to a clock repair or jewelry store to have this done, or you can also purchase the MSet device yourself. (Although the device cost over $600 USD.) I hope this was what you were looking for. Best wishes!
It is hard to say for sure, but several of the smaller minicomputers of the 1960s (e.g. DEC PDP-8) could and usually did sit on a desktop. However the term "desktop computer" was not created until the microcomputers of the 1980s and did not come into common usage until laptop microcomputers computers became widely available in the 1990s.