diff --git a/include/param_prj.h b/include/param_prj.h
index 262d41e..6731ea7 100644
--- a/include/param_prj.h
+++ b/include/param_prj.h
@@ -124,7 +124,10 @@
 #define CAT_CHARGER  "Charger"
 #define CAT_COMM     "Communication"
 
-#define VERSTR STRINGIFY(4=VER)
+// SKUDAK customization suffix: bump on every Skudak-side change so the OpenInverter
+// web UI shows a distinct version (e.g. "4=1.19.R-S1") and we can visually confirm
+// the right firmware is flashed. Match the stm32-sine -S<N> convention.
+#define VERSTR STRINGIFY(4=VER-S1)
 
 /***** enums ******/