Are there native harmonized code fields / subtabs on the item record?

Yes, there are. Here's a quick screen grab of what they look like on our Inventory Item form, under the International Shipping subtab:

I found the Schedule B portion when enabling International Shipping in Setup -> Accounting -> Shipping. I don't currently see where the Manufacturer portion comes from.

Perhaps a difference in options under "Enable Features" at the account level? We have some things like Landed Cost and Advanced Inventory Management turned on. Not sure if those fields are associated with either of those, but I suspect it something like that. I checked the field ids and none of the ones shown in my screen capture are custom fields, only native.

Thanks Dan