They are the most accurate drawings available drawn up at the time of construction of the ship. They are the final draughts of the ship in the design process and are meant to represent the details of the ship as she actually was on entering service and they thus provided a permanent reference for use by the Dockyards and the Admiralty when refits were undertaken. Sometimes also refit modifications where drawn if there was time, sometimes over the top of the as fitteds or more often on seperate sheets, however alot where never drawn or have been lost.
The responsability for drawing up the "As Fitted" plans was delegated from the Admiralty to the contractor in use and in the case of Exeter mine show Devonport (which is Plymouth) 1930. They also vary in small ways depending on that contractors drawing preferences and from draughtsman to draughtsman.
For big ships the scale of the drawing is usually 1 1/8inch = 1ft which is 1:96 and for destroyers and smaller vessels at 1/4inch = 1ft which is 1:48, exceptions being some coastal craft are drawn in 1:24.
They do not fully show a ship as in wartime condition though which for modellers is the constrait.
The "As Fitteds" plans which have survived the great purge in the British shipbuilding industry and Admirality/MoD dumping are now housed at the Brass Foundry, Out-station for the National Maritime Museum, Woolwich Arsenal, SE London. It's free to go and view them but expensive to get them copied.
Here is a photo of the building they are housed in which incidentially houses the largest ship plan collection in the World:-
