Cruchley's Reduced Ordnance Map of England and Wales
Folding version of the Cruchley's atlas of that name, a lithographic transfer from Cary's Improved Map of England and Wales (1832).
Sheet 27 only has north-west Essex (plus most of Hertfordshire and pieces of other counties); the remainder of Essex is on sheets 28, 19 & 20. The map detail goes to the edge of the sheet.
Displayed scale of 6 miles = 77mm, or 2.0 miles/inch, ie 1:126,720.
More details on Cruchley's maps are on the Publisher's page.
On the railways the lines from Loughton to Ongar and from Shelford via Haverhill to Sudbury are present (both opened 1865), but Audley End to the Saffron Walden line is absent (also opened in 1865). The Bishops Stortford to Braintree line is shown in blue (meaning proposed, as others are in red?) that opened in 1869, plus a line from Ongar to Dunmow that was never built.