As a historian who has been studying the History of the Penthyn Estate for the past 40 years, the dating of the "1841" map in your public area, from Gwyn, is clearly wrong.
It contains the Chester & Holythead Railway beyond Bangor Station westwards, and the Brittania Rail Bridge over the Menai Straits, so is clearly after 1848.
However it does not include the Port Penrhyn Standard Gauge Branch Line, so is pre 1852.
However finer details of the map show that it is likely to have been drawn in the spring of 1849, with publication in the following year. This mainly based on details of some of the houses that became occupied for the first time during late 1848.

Eric Foulkes.

Hi Eric,
Thanks for your comments with which I do not disagree.
The map is ISBN 0 71534848 5. Second impression 1980. Redwood Burn Ltd. The cartographical notes are by DR. J.B. Harley, 1970 and go into considerable detail on the development and revisions made to sheets 77 and 78. For brevity (but maybe not clarity!) I attributed the Ordnance Map Office date as printed on the bottom, right margin (Colonel Colby, Sept. 30th.1841).
Harley states "We may sum up by saying that the map published in facsilmile is a composite document embodying material from a succession of dates. The engraved maps of 1840-1(based on surveys originally executed and revised between 1816 and 1824 but also extensively revised in the late 1830's) were subject to relatively little revision apart from the insertion of Llandudno in the early 1860's, and of railway developments on a succession of electrotype plates"

Hi Gwyn,
Many thanks for your comments.
You might like to know that the First Series of all the 1 inch O.S. maps for England & Wales have been published in book form, 8 volumes in all. They are expensive to buy, but should be available at the Main Libraries at either Caernarfon or Bangor. If not, then contact Liverpool Reference Library, where volumes 1-5, and 8 are on open shelves in "Picton", whilst vols. 6&7 are in the Record office (available onlly to "authorised" people -you will need a Driving Licence toi see them!) However they will, I am sure, copy thenm on receipt of a letter. If not I willl get a copy, somehow. The Penrhyn Quarry, Bangor, etc. are in "O.S. Maps of England" and Wales, The Old Series", published by Henry Margary, of Lympne Castle, Volume 6, Wales, Sheet No. 12, made between 1816 and 1820. If you make a visit, the book gives further details (at some length!) of the dating of the features, and the amendments.

Regards,

Eric.