Thanks for covering for me whilst I was on Holiday! Yes, this is the disc plate from the vertebra of a cetacean. I have just been taking another look at the publication of Beneden en Gervais from 1880 and taking some measurements of the vertebral discs. Your disc is 77mm, Bottlenose Dolphin are about 57mm, White Beaked Dolphin about 52mm, Pilot whale 107mm and Sowerby's beaked whale around 80mm. Orkney is a frequent place for strandings of both Sowerby's and Cuvier's beaked whales.