Yes, Dolley quoted someone in a paper who referred to a "purple solution" that had been used at Soho to immerse the blanks in to produce this bronzed finish. It's surprising that no official bronzed proof coins have been made since the mid 19th century, given that that was the finish of choice for the first half of the 19th century. I wonder why they went off it?
I've got a P1335 1806 penny which looks identical in appearance to Taylor's golden bronze finish - this series (KP33 or 34?) is also plain edge and in a range of metals on rusted usually underweight blanks from over-polished dies. So some pointers to non-Soho restrikes perhaps, though the gilding on the gilt specimen in Gregory II was good quality, and the bronzed one in Gregory I (which I also have ex Alderney) is far more bronzed-looking (ie dark chocolate) in the traditional fashion though.