Hooray, the OSHPark PCBs arrived. Took some weeks to Germany with free shipping. They are perfectly purple, solder mask and silk screen are looking really good. Knowing that OSHPark does not officially support castellated holes, at least one side is looking ok. The other side is not through-hole plated at all, although the design is fully symmetrical. Nevertheless, should be ok for a prototype (solder blobs to the rescue!)