I think I've spotted the problem, it's a pre / post 1996 thing when the boundaries changed again. It would have been in Gwynedd before 1996. To confuse matters the names / jurisdictions of various organisations don't always match now. For example your link
http://www.themodernantiquarian.com/post/99532/links/pen_y_dinas.html
shows it's seemingly looked after by the Gwynedd Archaeological Trust.

I'll look into moving it. I think there might be others.

Thanks thesweetcheat,
TMA Ed.