Single on Sunshine label.
Melbourne singer Normie Rowe became Australia's top teen idol of the late 60s and the star of Ivan Dayman's Sunshine label. He had a string of hits in Australia from 1965, recorded four singles in London, toured Britain and America, and even inspired a tribute song, Norman-Normie. After he was conscripted to serve in Vietnam he never regained his earlier 'King of Pop' status, but he re-established himself as a respected performer, especially on stage, starring in such productions as Les Miserables and Annie.
On their first album The Magnificent Moodies, on Decca, entitled Go Now – The Moody Blues on London in the USA, released in July 1965, about a month after Normie Rowe's version was released.
Thanks to David Overett.
On Volume 2 of This Is Merseybeat, a 2 LP set recorded in Liverpool by John Schroeder using a mobile recording unit: see the 1963 story by Bill Harry from Mersey Beat magazine.
This recording appears to be the inspiration for Normie Rowe's version. Some sources cite the Merseybeats, but that band never recorded It Ain't Necessarily So. (The name of the album This Is Merseybeat has apparently been confused with the name of the band The Merseybeats.)
Ian & the Zodiacs were a Merseyside band most popular in Germany. See band history here [archived page]..
Musicology by Terry Stacey.
From the stage musical Porgy and Bess, written by the Gershwin brothers: words by Ira (1896-1983), music by George (1898-1937).
The original production of Porgy And Bess ran from October 1935 until January 1936 at the Alvin Theatre, New York (now the Neil Simon Theatre).
It Ain't Necessarily So was sung by John W. Bubbles (John William "Bubbles" Sublett 1902-1986) in the role of "Sportin' Life".
(Buyer beware! John W. Bubbles does not appear on Decca's "original" cast album [Amazon link]. It was made in the early 1940s "with members of the original cast".)
Single on Liberty (UK).
Texas-born James Marcus Smith, who changed his name initially to Jett Powers and then to P. J. Proby, had worked in the LA pop scene from 1957 as a versatile singer and songwriter. His deep, mannered vocals, often on reworkings of standard songs, benefited from skilful arrangements and production by the likes of Jack Nitzsche. Proby was always more popular in the UK – where he was based for some time in the mid-60s – and in Australia than in his home country.
References: P. J. Proby page at British Beat Boom; P. J. Proby article at Wikipedia (partly unsourced); P. J. Proby UK single discography at P. J. Proby – Blue Velvet.
Further reading: My Only in Oz (2) P.J. Proby – Mission Bell at PopArchives: The Blog. Michael Lane Heath, Get Hip to My Conflagration at Perfect Sound Forever.
Thanks to Terry Stacey for version alert and links.