unfamiliar with 7zip archive - Help!

[C.Tresenriter]

I have a two part archive that doesn't want to unpack.
I have P7zip, P7zip-full and P7zip-rar installed but the files
"somefile.7z.zip" and <anotherfile>.7z.Z01" will not open in Nautilus
"extract here" nor does "7z e <eitherfile.7z.*>" at the command line.
It looks to me as if the first file was compressed twice but
"unzip ...." reports that it's not a zip file.
I've also tried it under Windows, but get the same error each time: "An
error occurred while loading the archive."
The sender of the files tells me they open for him.

I've never seen these extensions before and uncle google has been no
help.