A320 Technician reported aircraft had wrong blocker doors installed on number two engine.

Narrative

Aircraft X arrived to Gate XX. During walk-around; found air gap around Blocker Door #3 on #2 ENG INBD Thrust Reverser (T/R). After further inspection it was discovered that a #5 Blocker Door was installed in Position #3. #3 Blocker Door part number is 31-XXXX-X-XXXX. See IPC (Illustrated Parts Catalog) REF 78-XX-XX-XX.Only Blocker Doors #2; #3; and #4 are the same part number. Blocker Door #5 is [a] different part number. Blocker Door #1 is [a] different part number.Further investigation revealed that either #1 or #5 Blocker Door can be installed in #2; #3 and #4 position. But #2; #3 and #4 Blocker Doors can't be installed in either #1 or #5 position. Unable to find when last maintenance was done on this thrust reverser half or blocker door. This may be a vendor issue installing blocker doors in wrong positions at the time of thrust reverser half build up. Need to pay better attention on building thrust reverser up...never rush or get in a hurry; do the job correct the first time. Safety should always be our #1 priority!!!!!!!