Oryx retweeted a tweet of Mehdi H. reporting that the Iranian Army displayed a surface-to-air defense system in their annual September 22nd parade in Birjand called Damavand, a device that is said to be actually a fake/dummy S-300.
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Iranian Damavand air defense system, said to be a fake/dummy S-300 (Picture source: Mehr news agency)

Dummies of all kinds is a trick that dates back to World War 1, the widest use having been implemented by the Allies in southern England in 1944 to lure the Germans about the actual gathering of forces in view of landing in Normandy on June 6, 1944. This kind of deception lost interest in the post-war ear, it seems, only to regain interest in recent years.

From afar, this British dummy M4 Sherman tank would be easily fooled for the real M4A1 Sherman tank it stands next to (Picture source: U.S. NARA/111-SC-216202-001)

The Russian army displayed various dummy systems like its Tor-M2-DT Arctic air defense system during its Armya defense forum and exhibition held annually in the Patriot Park, between Moskow and Kubinka.

It seems the Ukrainian army successfully uses decoys to lure Russian drones, so causing the waste of numerous expensive missiles or other artillery projectiles to destroy costless targets.

Regarding the Iranian army, no significant report is known mentioning the use of decoys. Damavand, as the name of the illustrated so-called dummy S-300 air defense system is called, is already known as the name of an Iranian navy frigate of the Moudge class, also known as Jamaran-2. One may wonder why dummy missiles would be loaded on actual truck chassis instead of being totally built as a cheap - inflatable or not - decoy.

Dummy Tor-M2-DT displayed at Armya 2018 defense exhibition near Moskow and Kubinka (Picture source : Army Recognition)