> AFAIK the zip is invalid - it appears to be coded as a spanned zip,
> but with only 1 file.

This may not be entirely clear cut. On a quick read, I find the reference document quite confusing on this point:

Spanned/Split archives created using PKZIP for Windows
(V2.50 or greater), PKZIP Command Line (V2.50 or greater),
or PKZIP Explorer will include a special spanning
signature as the first 4 bytes of the first segment of
the archive. This signature (0x08074b50) will be
followed immediately by the local header signature for
the first file in the archive.

A special spanning marker may also appear in spanned/split
archives if the spanning or splitting process starts but
only requires one segment. In this case the 0x08074b50
signature will be replaced with the temporary spanning
marker signature of 0x30304b50. Split archives can
only be uncompressed by other versions of PKZIP that
know how to create a split archive.

The signature value 0x08074b50 is also used by some
ZIP implementations as a marker for the Data Descriptor
record. Conflict in this alternate assignment can be
avoided by ensuring the position of the signature
within the ZIP file to determine the use for which it
is intended.

Cheers,
Liviu