The history on some of these older models is a little hard to come by. They weren't imported to the US back then, not alot was known about them.
I looked up one source and couldn't find a model P28 mentioned. It did say there was a P26, could you have mistaken the 6 for an 8? It also said the P26 was chambered in .380.
These are interesting old guns that have seen alot of history and been around the world alot more then I have. I wish these guns could talk. They'd have some stories to tell.