According to its serial number this helmet should be placed in the period of time when Siebe & Gorman became Siebe, Gorman & Co. (1881). The good news is that it can be completed with either breastplate, these breastplates had the text stamped directly into the copper. I would sell this bonnet 'as is' but would also be interested in buying the breastplate for it. Price on request.
Rare old bonnet of a Siebe & Gorman or Siebe, Gorman & Co helmet