The 1920s Macallan output was mostly supplied to blenders as a 'top dressing' single malt for blended Scotch, with the few remaining casks maturing for longer than usual.
The 1930s saw coal supplies fall due to increasing industrial activity so the distillery reverted to drying barley with peat fires. At the same time sherry casks became harder to source due to the Spanish Civil War.
The 1940s brought a shortage of sherry casks during the Second World War, and the distillery was forced to use refill casks more often.
The 1950s brought greater availability of sherry casks following the disruption of the Second World War, and the distillery was able to revert to using only first and second fill casks.