Regardless of where you place it, put a small recognizable and unique mark on the products you make. When you get rejects sent to you, you will know if they are yours or not.
Mark your products. You want to know your people are handling and guaranteeing the product you made and not by someone else. Add a unique company mark on your product so you are identifiable. If you do not have anything that identifies your product from your competitors, decide upon one and start using it.
Make your mark unique and difficult to copy. Your stamp does not have to be expensive but do make it unique and difficult to copy. You may be repairing competitors’ products. How would you know unless you have something on the item that indicates your firm’s identity?