Thanks for the link, although I was already aware of the content in GENUKI.

I'm still awaiting a reply to the email I sent (as recommended) to Gwynedd Archive in Caernarfon that as Monday was a Bank Holiday that's no surprise.

In the meantime it seems most likely as that the change happened in 1889 when the Counties were set-up but this was overlooked/ ignored by the 1891 Census Enumerator for Nantmor