[link to original on tumblr]
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I can't change the tone of the organ presets on my keyboard (an-other reason why getting a Nord - with physical drawbars! - would be such an advantage), so the organ tone doesn't match that well.
The bass during the repeated part at the end (which I played only part of because I don't know the whole mellotron part) is more complex than what I played, but I haven't figured out that part perfectly yet. This is actually a lot more than I thought I was going to do when I said I figured out some of the bass part. Mostly I was talking about the staccato notes and the long phrases that replace them later (one of them extends two whole octaves, up to the twelfth fret G), but then I kept learning more of it.
In Deep is the only Argent album I have as a record, so I can refer to the record sleeve for publication data (I'm not always too confident that what I find on the internet is right). The record credits "Losing Hold" to Argent and White:
According to the Zombie Heaven liner notes, Argent and White wrote separately but starting with "Imagine the Swan" they shared the writing credit. Because of this ambiguity, I sometimes wonder if a song is an Argent or White song. Obviously, I'm not sure of this, but I would guess that "Losing Hold" is a Rod Argent song because it has some of his characteristic three-note chromatic phrases in the bass part. The staccato notes at the beginning are A G# G, over which are played A major, C# minor, and G major; the chromatic phrase starting from the root note of the first chord in the sequence is a very Argentian progression. Later, during the repeated section at the end, there's a three-note chromatic phrase of C B Bb.
The chord progression has some resemblance to that of "Hung up on a Dream" too. They both have a section where each chord is only one note different from the previous. In "Hung up on a Dream" it's G major (G, B, D) to E minor (E, G, B) to C major (C, E, G). Here, there's a section of F major (F, A, C) to A minor (A, C, E) to C major (C, E, G) to E minor (E, G, B) to G major (G, B, D). This sort of incremental progression is more visually obvious on a keyboard instrument, which makes me think it's Argent's song.