Quite right. I have a number of holders--all 9x12cm--from various pre-Zeiss Ikon makers: Voigtlander, ICA, Contessa-Nettel, Ernemann, Nagel (Kodak), and no-name brands. I also have numerous 9x12cm holders stamped Zeiss Ikon with the same numbers as their pre-merger counterparts, and then Zeiss Ikon branded holders with new numbers.
The double-sided pop-off wooden plate holders for my ICA Universal Palmos 275 (9x12cm) are marked 737/6. Those same holders stamped as Zeiss Ikon have the same number even after the merger. So some things changed and some did not.
This thread is as good an attempt as any, but it would be great to pool resources and have a comprehensive database, with photos and measurements, of the various holders and sizes. But who has the time?