.Zip with the z01s and z02s have never extracted for me using 7Zip, and since the trial version of WinRar works, I assume it's either something I'm doing wrong with 7Zip, or it simply doesn't handle those kinds of files.
I have done some tests, and subsequently some research. 7zip does
not handle multipart zip files created by other compression utilities including WinZip.
I apologise for misleading people by saying anything different. 2 quotes:
1. The official 7zip forum on Sourceforge
7-Zip simply cannot open split Zip files created by other compression utilities. I tried renaming the Zip files to the naming standard used by 7-Zip, but it would not open the Zip file (it thought there was a Zip file in the Zip file, and after extracting that the resultant Zip file could not be opened). I also tried to get 7-Zip to open a split Zip file created using the latest version of WinZip (using the old compression standard, not their new one) and it couldn't open that one either.
2. Igor Pavlov, the author and maintainer of 7zip (in 2010)
There are many types of multi-part archives: ZIP, RAR, CAB. I don't like ZIP's multi-part standard by some reasons.
I don't plan to support it near future.
Multipart ZIP archives created by the official WinZIP archiver give "bad archive format" errors with a number of the free extraction programs I have tried, which are, like 7zip, created by 'one-man-bands'.
I suspect that this may be connected with the way the simpler freeware programs create and handle their own multipart archives. Many of them, in effect, create one large zip file and split it into blocks at the final writing stage. Both Winzip and WinRAR do things differently and use, I suspect, proprietary algorithms and methods not available to the "working from home" freeware writers. I work in a sensitive finance department and for compression we are only allowed to use WinZip with 256 bit encryption for files to be transmitted, even across our own secure intranet.