The design profession is usually the entity that is responsible for the design of the weldment or the organization responsible for coordinating the manufacture of the product. So, if you are designing and fabricating the component and selling it as a finished "off the shelf" item, you are the Engineering Authority. If the item is designed by a third party for an "owner" and the manufacture of the product is subcontracted to a number of vendors, the company responsibile for all the subcontracting is considered the Engineering  Authority.

It may sound easy, but it is often a bone of contention as to who has the "Engineering Authority". The easiest way around the problem is to assume the entity that subcontracted the work to you is the Engineering Authority. Any changes have to be passed through "them" for approval. They in turn would pass the request for a change up the food chain.

Best regards - Al