Thanks to 78rpm-maniac for solving the riddle for me!
S-1, S-2, S-3 etc. are names for the different label styles of German Elvis releases. They were introduced by the well-known German Elvis collector and musician Helmut Radermacher in his 1985 book "Rillen Revue, vol. 1" (singles) and in the 1987 sequel "Rillen Revue, vol. 2" (promos, 78s, EPs, singles supplement) (both co-authored with Wolfgang Kühnel).
The S-3 label was used from Jan. 1957 - June 1958 and has the line "Master Recording of Radio / Corporation of America" to the left and right of the RCA logo, instead of "Trade Mark ® Registered / Marca(s) Registrada(s)" (= S 1) or "Trade Mark / ® Registered" (= S 2).