This is probably a really stupid question, for items is there a External ID field?

Yes there is. I'm not sure you can directly add Internal ID or External ID to the form, but what I've seen in the past is to add just a custom text formula field with the formula just being {externalid} then it will show whenever anyone brings up the form.

all record type has external ID. By default you cannot access it from the UI. You can set the external ID (unique) for the records vis CSV, To show it in UI use a custom field and source "external id" value from the native field.