Isn't this a bit simpler than you make it out to be? It came to you as a Hetchins. The record of sale to Hetchins by the maker and the (implied) rebranding as a Hetchins by Hetchins is part of the history of this frame.

IMO, it would be wrong to rebrand this frame as a Bob Jackson. You can/should preserve the history you've uncovered, including the factual information that the frame was built in the Jackson shop. Even so, it should have Hetchins decals NOT Bob Jackson.

I am a little puzzled by the frame. To the best of my knowledge no one was using braze-ons for brake cables and deraileur cables in 1972. At best, it was extremely uncommon. Heck, my 1975 Bob Jackson did not have either and few others did, as well. Even the really high cost Italian frames used Campagnolo clips for these functions. I suspect these were added later on. Perhaps, others will be able to confirm these were added by Hetchins and were part of making it a Hetchins. This puzzle is pure speculation on my part - really more of a question than an answer.