I never even knew that Spector didn't release it back in the day, just thought it was among his greatest ever achievements. Why wouldn't he just put it out? I've first heard it on Back To Mono and thought it must've been a huge chart topper.
It was never released on record in 1965 when it was recorded, but it was used as the opening theme for Spector's 1966 "The Big T.N.T. Show" concert film (patterned largely after "The T.A.M.I. Show from the 1964, which of course featured the BB's and used J&D's "(Here They Come) From All Over The World" as its theme: J&D were also the hosts).
The performers on "The Big T.N.T. Show" included The Byrds, Petula Clark, The Lovin' Spoonful, Bo Diddley, Donovan, Joan Baez, The Ronettes, and Ike & Tina Turner among others.
I found a 2008 Phil Spector publishing CD (EMIMP Presents: Wall Of Siound - Philles Records Volume 1) at Amoeba Records that had a newly remastered version of "This Could Be The Night," and it's a significant upgrade from the BACK TO MONO box. Let's hope for a remastered version of the BACK TO MONO box to match the sound on this publishing CD and the 2006 UK issue PHIL SPECTOR'S WALL OF SOUND RETROSPECTIVE.
Another MFQ album for BB fans to search out is their 1985 MOONLIGHT SERENADE LP (reissued in Japan on CD in 2001) which features them doing eleven standards and one original with Four Reshmen-style arrangements. Great stuff!