Thanks everyone for your help. I am feeling more comfortable that the “528” plates are correct for the 911.
I suspect that the “228” plate might have come from a 356, the “675” plate from a 1970’s BMW Bavaria, and the “886” plate from a 1986 BMW 323i.
My father-in-law told stories about buying a tiny two-cylinder car for an extended European honeymoon (possibly starting in France) in ~1953. Do any of the black-background plates look like they could have come from that car?