Genuine Bertoia Diamond Chair or reproduction?  

Yes
Authentic. Did you buy them?

Old Diamonds are nearly always authentic
...because they were historically too cumbersome and expensive to reproduce and sell at a discount.
That's no longer the case, but usually the welds around the top edge are on the underside of the top rod - or sandwiched between a double rod.
Either way - easy to spot. Yours look good. Powder coat and enjoy.

Thanks for the prompt feedback !
Thanks very much for your help and prompt comments.
I was confused because of the absence of rail on the bolts connection the seat to the base, but I am far from being an expert.
They are apparently from 1991, which would tally with your comments regarding old Diamond models.
How much would a (Rilsan) powder-coating cost (per chair)?

Rilsan coating
D-sign
Your first question was perhaps difficult to answer using the internet. Forum members were able to help.
The cost of Rilsan coating can be ascertained by contacting one of the many companies listed on the "internet". I have attached a link for Advanced Industrial Coatings (found on the internet). Make some type of attempt to do your own homework.
http://www.aic-coatings.com/#!rilsan-nylon/c195t