Guys what is giving me a considerable headache is the lack of tail number information online on non-US flights.

Some of them I have been able to trace back through libhomeradar/acarsd (this is a unix project of amateur flight enthusiasts that pick up plane beacon signals and log them), but there are quite a few tail numbers I can't retrace.

Do you know any other way or would it be worthwhile mailing the airlines with the question to have a peek in their history books?