Starting in version 9.0 online, the following entities have a read-only field called productnumber:
- Quote Product (quotedetail)
- Order Product (salesorderdetail)
- Invoice Product (invoicedetail)
The field is a read-only copy of the Product ID (productnumber) field from the related Product.
This https://docs.microsoft.com/en-us/dynamics365/customer-engagement/web-api/salesorderdetail?view=dynamics-ce-odata-9 shows the field for Order Product.
Interestingly the field is not available on Opportunity Product nor is it available in Dynamics On-Premises.