You could have a wander around http://www.british-history.ac.uk/ .

Click on the maps tabs a go from there. This link [
http://www.british-history.ac.uk/map.aspx?compid=55181 ] gives an
index for Caernafonshire which looks like the kind of thing you mean.

If not, David Archer's Indexes are the best paper source [
http://www.david-archer-maps.co.uk/content/ordnance-survey-books-david-archer-pu\
blications
].

Lez