S2 is comparable to the WaA marking, used before the WaA. You see these on cases that were in use years before the start of WW2. In my collection I have these markings on the 3,7 cm PAK (S2, 1935 and B78, 1935), 3,7 cm Flak18 (B78, 1935) 7,5 cm iG18 (S2, 1934), 8.8cm Flak18 Aushilfskartusche (S2, 1935), 10,5 cm lFH18 (S2, 1935), 15 cm sFH13 (S2, 1935). So for 1934 (K) and 1935 (G) you see these markings, but in both years I also have cases with WaA. Never investigated deeper how this was introduced and stopped.