philmh: steve winwood's separate merkin rights arose from the days of separately-licenced deals individually licenced for brit/bpr/island productions antedating deals for the island label as a whole, right back to his spencer davis group days.
by the time he launched his career as a fully solo artist early-mid seventies from about "steve winwood" and "arc of a diver" on°, in parallel with the late incarnations of traffic tours, his stature was such that he (or his agent) could negotiate separate solo deals with music corps and musicbusinessmegacorpses for the different historical markets - and did so: winwood's now a landowner in his own right, as well as a musician who can choose when and with whom he tours; - and more power to his elbow!
° - the "john barleycorn" project was originally intended to be his first, major solo album, but it metamorphosed in the recording sessions into "the traffic reunion album" - and very successfully so
I've given up on Billboard; I went to 45cat instead, and probably the last Atco-distributed Island single seems to be By All Means, which has a date of March 1990 (the label has a remixing credit for January 1990, after Island had moved to PolyGram elsewhere in the world), so in the absence of any other information, I am going to assume that US PolyGram started distributing the label in April.
I'm trying to find out exactly when US PolyGram first got Island, but so far the Billboard archives in Google Books aren't being very helpful. It could well have been as early as January 1990, as that is when PolyGram here in Australia got the label (and A & M, which PolyGram bought at the same time the previous year). The 1985 issue of that Propaganda album was actually via Atco with catalogue number 90288-1/2/4, but unfortunately the wording "released September 1985" that appeared on the cover of that version also appeared on the PolyGram version, leading to inevitable confusion.
US PolyGram got Island no later than April 1990, as they released Marianne Faithfull's BLAZING AWAY that month. I see some European issues with PolyGram numbers dated earlier than 1990, but I think those might be correct, as I believe France and possibly some other countries had Island via PolyGram before the sale (not Germany, though - I think they still went through Ariola/BMG for a little while after the sale, presumably due to the terms of a previous contract).
Aren't the cd's that start with 4122 and have a phonogram cat# in the Island discography dated wrong? must be after may 1990 when Atco distribution stopped...
Atlantic/Atco started distributing the bulk of the Island rock/pop/soul catalogue in the US in 1982; exceptions being every reggae act whose name wasn't Bob Marley being released on Mango, the more marginal rock/pop/jazz product released on Antilles, and Steve Winwood who somehow had a separate US contract with Warner Bros.
Track 06 with New Voices of Freedom.
Track 12 with B.B. King, guitar and vocals; Rebecca Evans Russell, Phyllis Duncan, Helen Duncan, backing vocals.
Track 11 with Bob Dylan, backing vocals.
Track 13 with Brian Eno, keyboards.