9x12cm is the same as 3 1/2 x 4 3/4 inch. The film pack adapters were made to be used with film packs. A long discontinued way to package and use thin sheet film.
Originally Posted by Keenevision
The Zeiss Ikon 3 1/4 x 4 1/4 inch plate holders that I've seen are a kind of "reducing" back. They usually fit the typical 9x12cm plate cameras, but hold the 3 1/4 x 4 1/4 inch plates that were more common in the U.S. at the time.
The glass plates that you got are most likely 6.5x9cm and wouldn't fit in any of these holders.