If my understanding is correct, big players like Hitachi would be producing for self-consumption, not for selling to other companies. So, in my opinion, no. The broader question is would the backward integration into the in-house manufacture of bushings be carried out by more and more industry players, which may not be given that it is a small (in terms of overall % of value) component of transformers, yet very critical to their operation and needs specialised manufacturing. Check out the post of the master researcher Phreak.
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