When was the first publication of the abbreviation RSA (Rivest, Sharmir, Adleman) because it does not appear in Martin Gardner’s article of 1977 which is at the following url: https://simson.net/ref/1977/Gardner_RSA.pdf
Nor does it appear in A method for obtaining digital signatures and public-key cryptosystems (Communications of the ACM Volume 21 Issue 2, Feb. 1978 Pages 120-126), which was Rivest-Shamir-Adleman's original publication. It is hard to pinpoint the watershed with such obvious abbreviations, but MathSciNet shows Blakley-Blakley Security of number-theoretic public key cryptosystems against random attack I (Cryptologia 2 (1978), no. 4, 306–321) as the earliest occurrence. The trio founding RSA Data Security, Inc. in 1982 (now RSA Security LLC) probably solidified the usage.