There's no question to my ear [[and from working with nearly all the Crystals and Darlene over the years) that the released version is La La.

That being said, In a new transfer of the mono mix made for one of my cd set's, when highly processed through radio airchain filters - - I am in fact wondering if Darlene might be there in the background harmony parts. on the released hit version. I know La La doubled herself on the hit version - maybe some of the background track still had some of Darlene's vocals buried in the wall of vocal harmonies, particulary the phrasing of "Da Doo Ron Ron" the Crystals did, vs what sounds like a "Da Doo Run Run" lower voice in each repeat section of the hook that perhaps the Blossoms may have done?

I certainly love Darlene and give her 1000% credit, but the hit version is very clearly LaLa. Maybe it's possible some of Darlene is still buried in there on the mono release version - the Blossoms voices were so distinctive, I'm now thinking it may be possible Darlene's voice is buried in there on the "Da Doo Run Run" phrasing parts before the drums.

I don't hear her parts on the background multi's I've heard from the song, which makes me wonder if it's left over from some part of the mono mix wall?

Maybe....