My good mate Peter Valicek sent along some images of a couple of unfamiliar engines which were found by a Swiss collector friend of his.  They are clearly the same engine with different anodising. At first glance they look like Metro 2.5 cc diesels from Germany - the round exhaust ports are usually indicative of that marque. However, in other respects there are significant departures from the design arrangement of the Metro 2.5 cc as it has generally been depicted - for one thing, the crankcase casting is somewhat different, as is the needle valve alignment. It’s quite possible that these are previously-undocumented variants of the basic Metro design, but I've been unable to confirm this - they could be Metro-influenced engines produced by others. Can any reader shed any light on these engines?