Duffy's Nucleus were essentially synonymous with Duffy Power, the good but obscure British R&B singer who interacted at various points in the 1960s with Graham Bond and future members of Cream, Pentangle, and the Beatles, as well as John McLaughlin. In 1966 and 1967 he was the head of a temporary group called Duffy's Nucleus, which also included McLaughlin, bassist Danny Thompson and drummer Terry Cox (later to be the rhythm section for Pentangle), and drummer Red Reece (who was replaced by Cox). There was just one single billed to Duffy's Nucleus, "Mary Open the Door"/"Hound Dog," in January 1967. "Mary Open the Door" is a fine soulful blues-rock Power original, and "Hound Dog" a less inspired cover of the much-done early rock classic. With McLaughlin, Thompson, and Cox, Power did some demos for producer George Martin's AIR company, which impressed Andrew Oldham. Whether or not they were the work of "Duffy's Nucleus" is moot, since they didn't come out.
Power must have liked "Mary Open the Door" a lot, because he recorded it three times in the 1960s. The best and most rock-oriented take came out on the 1970 album Innovations (although it had been recorded in the mid-'60s), which has subsequently been reissued in different packages. The Duffy's Nucleus version is differentiated from the Innovations one by the presence of horns and female backup singers, and in 1969, he did an acoustic rendition for the Spark album Duffy Power. The Duffy's Nucleus version of "Mary Open the Door" and its "Hound Dog" flip side are available on the various-artists anthology The British R&B Explosion '62-'68, and the Duffy's Nucleus version of "Mary Open the Door" is on the Duffy Power compilation Just Stay Blue.
The discographical intricacy doesn't end there. A somewhat more energetic alternate take of "Hound Dog" surfaced on Just Stay Blue. What's more, Just Stay Blue lists the personnel on the Duffy's Nucleus version of "Mary Open the Door" as being Power, McLaughlin, Jack Bruce, and Ginger Baker, although it lists the personnel on "Hound Dog" as Power, McLaughlin, Thompson, and Cox.