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This board plugs into the OCXO connector on the counter. There are two jumpers (JP14 and JP15) on the main board near where the OCXO is plugged in that need to be changed to select the OCXO. See the last two pictures (from the 6680 manual) for schematic and location on the board.
When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.