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Handling Codes in Sheet Stock Library in Workbook Designer

We are using Microvellum Toolbox 2025 (BSB) build 25.1.1218.641 with library Foundation 22.0203.
Sheet stock materials have the handling code filled in with the inventory number.
In the Workbook Designer, I'm looking for a Material Handling Code column on the Sheet Stock Library tab to view these inventory numbers, but I can't find it.
Does anyone know where the Material Handling Code is stored?
Thank you for your answers.