His first point is who else could have made them? Dixon ceased production in 1865, before the introduction of orange lustre. Moore & Co closed in 1874, which is just within range (we know Moore & Co produced orange plaques – see April 21 posting). The other contender is Ball's Deptford Pottery (Dick Henrywood tentatively suggests this attribution in his article 'Poor Man's Pictures'). Baker writes that in November 1897, at an auction of Scott's effects, copper transfer plates were bought by Ball's Deptford Pottery.
Ian says that imperfections in transfers, which appear on Scott marked wares and orange round-cornered plaques support the attribution. There is a transfer imperfection that appears in my three Bretagnes, which shows that they all came from the same copper plate. Take a look at the three pictures below (pink plaque left, orange plaque centre, and plate right). There is a vertical white scratch on the right that cuts through the top row of gun placements (click on the images to enlarge).
Looking at these details though, that seems unlikely. As copper plates are used over and over, the engraved lines become weaker. If the orange plaque was made post 1897, when the plate had been around for 42 years, we'd expect the transfer to be fainter. In fact, the details on the orange plaque are far crisper than those on the Scott marked plate.
All things taken into consideration, a date of around 1870 and a Scott attribution seem the most likely.