The letter "A" on the graphs from Crudace et al., 1974
(http://adsabs.harvard.edu/abs/1974ApJ...187..497C) is used to 
denote the element

                   Argon

Until 1957, Argon was described using "A"; after that time, 
its official abbrevation was changed to "Ar".  However, some
people continued to use the old abbrevation ... for some
twenty years or so, it seems.