I noticed I still have a copy of ziics on my computer -- I never throw anything away. The doc files suggest that the ZIICS chess sets were in a format unique to ZIICS, but that the ones called "Fritz" were converted from Fritz -- whether with or without permission I don't know. Here are some snippets from the ZIICS doc files:
If you examine the list of files which are included with ZIICS, you'll
notice many chess set file names. The extension of the filename designates
the video adapter, and size, of the set. V32 is the smallest VGA set; V56
the largest. H40 and H56 are sets used for Hercules monochrome; and C40
for CGA.
ZIICS includes different styles of chess sets for VGA: BOOKUP.V40 by
BookUp CHASST.V40, the set used by the Chess Assistant programs;
SOLID.V40 is derived from the GIICS chess program. Other sets were
created by ZIICS users.
...
The program SHOWSETS.EXE displays ZIICS 1.14 VGA chess sets. It also
shows the EGA/VGA colors available with ZIICS 1.14.
The file extension tells the video adapter and the size of the set.
V = VGA, H = Hercules, C = CGA. The number is the width of each
square in pixels. This ranges from 32 to 56. EGA can use V32 and V40
sets, but the squares are not square.
BOX40.V48 uses the same pieces as SOLID.V40, but uses a 48 dot square.
There are four styles of VGA chess sets. BookUp.V40 is from graphics
in BookUp 8.5 and is used with their permission. Fritz.V40 is from
the Fritz chess program. MB.V32 was drawn by mb, a player on the US
ICS. The other sets with similar names are magnifications of the
above sets. The other sets are derived from the GIICS VGA set.