I told you that none of the references I've found from the 19th century refer unequivocally to a specific product. There's one that might possibly from the 1890's, but none that unequivocally point at a beer really called Nut Brown Ale.
This is the earliest one I can find in the British newspaper archive:
Western Times, Monday 27 December 1900, page 1.
St. Anne's Well, a modestly-sized brewery in Exeter, were an innovative bunch. They were one of the first British breweries to produce a Lager in the 19th century. They also seem to have been a Nut Brown Ale pioneer.
Judging by the price and description, "A" looks like an early Dark Mild. When did St. Anne's Well start brewing "A"? Luckily, I've earlier adverts from them. I extracted this information from one:
|St. Anne's Well Brewery beers 1891|
|beer||price per barrel||price (per gallon)||price (per doz) pint||size|
|S.G A specially brewed, Pale Mild Beer||36||12|
|St. A. P. St. ""ANNE'S PALE ALE."" A light, Pale, Bitter Ale of excellent quality, highly recommended for famile use.||42||14|
|E.I.P. St. Anne's ""East India Pale Ale"" brilliant, and very superior quality Ale.||54||18|
|H.B. St. Anne's ""Home Brewed,"" a Mild Ale, but not Pale.||48||16|
|St. A. XXX A Mild Ale, of the Burton character, of very superior quality.||48||16|
|St. A. XX A Mild Ale, lighter than XXX, but of the same character and excellence.||42||14|
|A St. Anne's Ale, lighter than H.B., but of the same character. Recemmended for general family use where a Pale Ale is not required.||36||12|
|S. ST. ANNE'S STOUT||54||18|
|ST. ANNE'S LAGER BEER||3s 6d||screw Imperial pint|
|ST. ANNE'S PALE ALE||3s||Imperial pint|
|ST. ANNE'S DINNER ALE||2s 6d||Imperial pint|
|ST. ANNE'S STOUT||3s||Imperial pint|
|Exeter Flying Post - Saturday 26 December 1891, page 8.|
Now isn't that interesting? "A" is described as a lighter version of Home Brewed. Which itself is described as "a Mild Ale, but not Pale". Making it sound very much like an early, strong Dark Mild. Note that both beers are draught.
What I think we see are the beginnings of both Dark Mild and Brown Ale. But why did Home Brewed and Brown Ale become almost exclusively bottled beers? Was it because of the technological changes that allowed non-deposit bottled beers? And why wasn't bottled Dark Mild called Dark Mild?
Loads more questions and no real answers.