The attached images of this very neat 2.5 cc cross-flow loop-scavenged diesel were sent along by my valued friend Peter Valicek of the Netherlands. The engine bears no markings at all to guide us. However, Peter is 99% sure that it's of Czech origin. He says this because it has two features which are almost definitive - an aluminium alloy contra piston and a screw-in backplate with no slots to facilitate tightening using a suitable tool. I agree with Peter that taken together, these features are pretty indicative, although there's always the possibility that the engine did originate elsewhere.
Peter and I have both searched our extensive reference libraries and come up empty on this one. If any reader can do better, please get in touch!!