70 Challenger Front Bumper Rubber Retainers

[TWS]

Do we know when they began using the metal retainer brackets to hold the front corner rubbers on the 70 Challenger front bumpers?  The problem was solved when they went with the notched bumper with corresponding self-locking rubber design in the 71 model year.  Some kind of a 70 mid-year change when they discovered the rubbers coming loose?  Our October 14 '69 SPD car did NOT have them.  Thoughts or perhaps factory documentation?  Thanks!

[750-h2]

My 6/Dec/69 car has them?