Railway sub-offices

[jcopeland]

I'm trying to confirm the precise dates these started and ended. The article on the Railway Philatelic Group website says "The first ever Railway Sub Office was created in 1855 at Braco (Perthshire)" and "On 1 August 1905 all former Railway Sub Offices were re-designated simply as ordinary offices or sub-offices." while the article on the TPO & Seapost Society website says "The first post office to become so designated was Braco in Perthshire which became an RSO on March 1st 1856" and "the title of RSO was discontinued as from June 30th 1905." but neither give the original sources for their statements.
I am aware of an announcement in the Post Office Circular of 28th June that in the 1st July 1905 Post Office Guide "The letters S O will in future be employed to indicate both Railway Sub-Offices and Independent Sub-Offices" but this does not deal with the use generally. Was there another announcement in the Post Office Circular about the more general use of the term or are there other primary sources to confirm the date the use ended? Also can anybody provide details of a primary source regarding the commencement?
Incidentally in some April 1909 newspapers there was an announcement "In the case of certain small towns and villages it has hitherto been necessary for the letters R.S.O. (railway sub-office) to form part of the postal address. The Postmaster- General has decided that in future these letters need not be used."