As the club's $1.2 million redevelopment project continues, the letter has already caused a stir among some RSA members.
And it could result in the club's earliest known history being rewritten and extended by more than 20 years.
Club committee member Bing Crosbie said he was ''gobsmacked'' to receive a copy of the letter, which was recently sent to the Mosgiel club by Pat Young, of Dargaville.
Mrs Young (82) is the daughter-in-law of Ernest Young, who served with the Otago Mounted Rifles in Gallipoli and later became a founder and first secretary of The Taieri Returned Soldiers' Association.
Mosgiel RSA club officials said details of much of the club's early history had been lost, and club members had generally believed the club had been formed in the 1940s, after World War 2.
But striking new insights are provided by the letter, which was written on June 10, 1919, on ''The Taieri Returned Soldiers' Association'' letterhead, and sent to Mr Young.
The letter gave ''heartfelt'' thanks for his ''untiring energy'' and an ''unselfish'' approach which had brought the Taieri association to the ''satisfactory state at which it now stands''.
Mr Crosbie (84) was greatly surprised by the letter's contents, which suggest the club's history may be more than 20 years longer than previously believed.
''I sat there and read it three or four times and thought 'Oh God'.''
He immediately realised the club's early history would now have to be seen in a completely different light.
''We were thinking that it was after World War 2 that it really all got going,'' he added.
But the letter revealed a differently-named returned soldiers' organisation in the area, at a much earlier stage.
Mr Crosbie has helped co-ordinate the club's library and military memorabilia for the past 10 years.
And, after reading the letter, he is still scratching his head about details of the club's early history and wants to learn more.
He is particularly keen to discover when the Mosgiel Memorial RSA name was adopted, and the nature of any connections between the current branch and the former Taieri Returned Soldiers' Association.
He was unclear whether an early Mosgiel RSA body may have existed alongside a more rurally-focused returned soldiers' organisation, or if one organisation disappeared and was replaced by another, or there had been some amalgamation and renaming.
Mosgiel RSA vice-president Tony Mobbs said the letter was '' a big surprise'', had provided ''very important'' information, and had come at a ''very opportune'' time as the club's redevelopment neared completion.
And the 100th anniversary of the Gallipoli landing, on April 25, 1915, would be commemorated next month.
After serving as the first secretary of the Taieri soldiers' organisation, Mr Young, who lost his right arm in the Gallipoli fighting, left Dunedin after receiving land to break in for farming in an area north of Dargaville, later known as Struggler's Gully, in a postwar government ballot.
He later retired, and lived in Dargaville, where he died in 1963.
His parents, James and Hannah Young, lived at Dryden St, Mosgiel, and had had four sons, including Ernest, who all fought in World War 1 and survived.
Mrs Young, who recently found the 1919 letter folded in an old wallet, said it was ''wonderful'' that it had sparked so much interest at the Mosgiel club.
She recently won a Government ballot involving a privately-funded trip to Gallipoli next month, and is keen to trace any other relatives of Ernest Young living in Mosgiel and Dunedin.
She is particularly interested in whether any of these relatives plan to visit Gallipoli on Anzac Day this year.
People with more information about the early history of the Mosgiel RSA, or about Mr Young's descendants are asked to contact Mr Crosbie at the Mosgiel club or at crosbiebing@gmail.com via email.
