The first task for The Beatles in 1966 was to overdub or "sweeten" the soundtrack to the film "The Beatles At Shea Stadium" which had been filmed back in August of the previous year during their tour of North America. So, they gathered at CTS studios in London on January 05 1966 especially for this task. Most people were not aware of this session at the time.
George Martin supervised the recordings as the Beatles re-played in it's entirety the following songs from the film: "I Feel Fine", "Help!" and "Ticket To Ride". Paul added bass guitar to the following songs from the film: "Dizzy Miss Lizzy", "Can't Buy Me Love" "Baby's In Black" and "I'm Down". The overdub was due to a complaint of needing more bottom end for these tunes. John Lennon also overdubbed his organ part for "I'm Down" in order to fortify the performance.
The remaining tunes were also substituted from the original August performance "Twist And Shout" (which was replaced by the Hollywood Bowl Capitol Records recording of 1965) along with "Act Naturally" (which was replaced by the commercial version from the LP recording of the song"). This leaves only one tune that was left untouched : "A Hard Day's Night". The song has dialogue over the performance so in fact the entire Shea Stadium concert was never presented in it's true original audio form.
The Shea Stadium soundtrack has been released in various bootleg forms over the years with titles such as "Shea, The Good Old Days"/ "Shea/Candlestick Park" etc.