On the RS, they added several additional reinforcement bars that were not there in 1972 to meet the new crash requirements. The bars from the radiator support to the front fenders were also longer so the engineers did their diligence and testing.

These bars:

Left and right fender to the core support.
Left and right core support to the reinforced hood latch which was connected to the header panel.

My roommate in college had a silver 1973 RS Type LT Z28 that he bought used in 1976. It was a great car and he argued up and down with people that it was a Z28. He finally had stripes painted on it. It was a very fast car.

A 73 or 74 Z28 with a 4:10 gear swap was a fast car for that time for sure.

With these 11 unique support bars found on only the 73 RS. Very hard to find if you are missing them.

Yes the remaining bars are core support to the header along with the reinforcement plate.

Were the extra bars added starting on January 1, 1973 or from the start of the '73 model year production?

Not sure when the photo was taken but this also seems like a great color photo example of the factory finish on valve covers...

I believe the extra bars started in the beginning of production as they are in the assembly manual. 73 changed from a bow style headliner to a board style by the 3rd week of Feb.

I refinished mine and think they are close.