While the first detailed studies of faint structure and substructure in M31 did not appear until the early 2000s, there were a number of studies prior to this which are of particular note.
Baade & Gaposchkin (1963) were the first to comment on the strong warp present in the outer parts of M31's stellar disk. Baade (p. 73) notes that the disk signature is still present at a major axis distance of 2∘ but that the opposing sides of the disk have ‶swirled‶ off in anti-symmetric directions by a radius of 2.25∘ – an observation he speculates could be due to the tidal action of the Milky Way on M31. The existence of the prominent stellar warp was further confirmed by Innanen et al. (1982) who stacked digital Palomar Schmidt plates to reach surface brightnesses in M31 of µV ≳ 25.8 mag arcsec−2. Early H i studies of M31 also revealed a strong warp in the neutral hydrogen disk (e.g. Roberts & Whitehurst 1975, Newton & Emerson 1977, Cram et al. 1980). While the stellar and H i disks are warped in the same direction, the stellar warp appears to begin at a smaller radius than the H i warp and it exhibits a greater deviation from the disk plane; this holds true along both major axes but seems especially apparent in the north-east.
Walterbos & Kennicutt (1988) conducted the definitive study of the light distribution in M31 before the era of wide-field resolved star mapping, including a first quantitative exploration of the peripheral disk. They constructed multi-band images from digitised Burrell Schmidt plates and analysed surface brightness and color profiles across the disk. They also detected a clear warp, but speculated that the north-eastern warp, due to its faintness and extreme bend, may actually be a galactic reflection nebula and not a stellar feature associated with M31. This issue was settled by Morris et al. (1994) who resolved the stellar populations in the north-eastern warp (which they termed the 'Spur') for the first time and showed they lay at the distance of M31. Fig. 1 of Walterbos & Kennicutt (1988) shows beyond doubt that the north-eastern half of the disk is far more perturbed than the south-western half, and, in hindsight, one can even see a slight luminosity enhancement in the direction of the Giant Stellar Stream.
Equally influential were the first studies of M31's resolved stellar halo. Mould & Kristian (1986), Crotts (1986) and Pritchet & van den Bergh (1988) all presented colour-magnitude diagrams (CMDs) of small regions located at several kpc along M31's minor axis, a region perceived at the time to be dominated by pure halo. These studies revealed a moderate metallicity population ([Fe/H] ∼ −1) with a large metallicity spread, in stark contrast to the metal-poor halo of the Milky Way. The disparity of the halo populations in two galaxies that were otherwise considered rather similar remained a significant puzzle throughout the following decade.