This pattern of Shoulder Belt Plate (SBP) came into being in 1809 when the 84th were given the title of York and Lancaster. The plate is hallmarked sliver with a date of 1809.
The battle honour Peninsula wasn't awarded to the 84th until 1815 for the 2nd Bn and then 1818 for the whole regiment. The battle honour Nive wasn't awarded until 1818, therefore, as the hallmark is 1809, the scrolls must have been added to the SBP at a later date. This can be evidenced by the original Rose/Coronet/Garter being impressed into the plate whereas the 2 battle honour scrolls have been pinned and bolted onto the plate.
As a new pattern SBP was introduced in 1826 (again in silver) which in turn was replaced by a SBP made in gilt in 1830. It can concluded that this particular SBP shown was in use between 1809 and 1826.