I really don't know just how relevant this might be, but some Brandywine products are/were rebadged units from Time and Frequency Solutions in the UK, or perhaps vice versa. I'm not sure if TFS had an equivalent of the GPS4 but certainly both companies sold versions of the GPS8 and the TFS GPS8LN uses a Milliren 250-0504, a 5MHz SC cut OCXO, so perhaps the GPS4 might also use a Milliren oscillator? Nigel, GM8PZR Hi, Does someone happen to know what OCXO sits in the Brandywine GPS-4 GPSDO? It's pretty good sized OCXO, possibly double-oven. It has a screw on the side which I presume opens up to the trimmer position. If someone could share info it would be useful. Cheers, Magnus