I’m at work where I can’t see my Beatles Albums, but I’m pretty sure Billy Preston received playing credit on “Get Back”. And of course the Beatles used all kinds of session musicians, from their first album on.
Ike – I suspect album credits may be a contractual matter. Both horn players you mention were professional classical musicians. The on on “Penny Lane” may not have wanted to be associated with a pop record. As for the dude on “For No One,” either he insisted on the credit, or the record company offered it to him in exchange for a reduced fee.
Sam – Yes, Preston did get credit on “Get Back.” More than just credit: on the 45 the artist was listed as “The Beatles with Bily Preston” – making him as close to a 5th Beatle as there ever was. He was not given songwriting credit, though.
“The dawn of a new era is felt and not measured.” Walter Lord