There are several links that fit the Campy UN properly. The only wipperman connex that fits properly is the no longer made 10S1 model. The SRAM 10 powerloc, Forster superlink model 4 or a special KMC missing link made for the Campy UN chain will all fit.
These days there are two groups of inner plate widths that dictate the proper connecting link. The wider group includes Shimano, KMC and Wipperman. Sram and Campy UN are both narrower across the inner plates.