I have just read the Lens Vade Mecum on Grubb. The move to England was in 1918 and was probably concerned mostly his other interests. LVM talks quite a lot about the new design/engraving and gives some 5xxx serial numbers. It does say Dublin and Howard Grubb on the late models. His production was so small and the late lenses look well produced so I think he had someone else make these. The landscape aplanatic had a good reputation, so was still a saleable article in the 1880's.

Later:

Thanks for the first-hand information - quite a rare thing these days!