Good Day Matt,
Right that IR in oval cartouche appears to be John Robertson of Newcastle. This particular mark has been seen between 1795 - 1797. Upper case G in clipped cartouche is the Newcastle date letter for 1797. From my reading here had assumed that the lack of a city assay office mark took place between the end of the 18th on into the early 19th centuries in London and at least one other city on small objects.
Let's see what our other more knowledgeable members have to say.