Creating a manufacturer trading, Item class logic & validation for engineering items

Summary:

I am trying to add manufacturers for some items, that are in different item classes. All of them engineering items.

For one item class, the manufacturer option does not appear in the drop-down list for these items, whereas it is visible for non engineering items. Additionally, when I check some other items with the engineered flag set to 'Y' in another item class, I can see the manufacturer listed.

Can someone tell me where the setup/logic for this is?

 Thanks

David

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