The Nantlle area 1889 (published) OS map was surveyed in 1887, according to a note on the lower margin of the printed sheet. It is possible that there was generally a year's delay fro survey to print, as in the 1899 Nantlle survey 1900 print 2nd edition, though the 1913 survey was not published until 1916, probably due to obvious reasons! I'll look up my old paper files for you to look for Moelwyn dates; there might be something in them!