Have you seen this version of Finn Juhl FD133 "spadestolen"?  

The backrest is: 

Another perspective

And this is the original mark "France & Sons" on the seat of the chair

I have reviewed a France & Sons catalog published in 1958 and there is ad  of this chair but with vertical slats. That is, somehow, the firm sold this type of backrest structure. 

I haven’t seen this particular feature but France and Søn did change the designs over time. Especially when Poul Cadovius took over the company and they moved production in the late 60s, many details were rearranged, sometimes spare parts from one design would be reused for other designs as far as I remember. So it doesn’t surprise me to see a new version of the chair. Did you notice that there‘s also a version with a structurally sound backrest? 

Looks custom to me, especially since the slats fit nicely into the semicircles where the rubber loop coils would have wrapped around. The slats themselves look like they may not be teak, and they also don't look uniform, which makes me think they weren't machined. Not to mention they're nailed in. I'm struggling to think of any France and Son furniture that was joined by a nail!

@kyle-barrett So happy to see you back.

Agree with @kyle-barrett that the chair has been modified from original and with @herringbone that it's nice to see Kyle after such a long while!

@madruga, I'd be talking to the auctioneer about a refund at this point. The chair frame itself is original, but with the original loop spring supports removed and wood slats added as rigid replacements. If I'm not mistaken, it seems that this is also the case with the seat support as well as the back.

Maybe @cdsilva would share his thoughts on this?

You guys are too kind for someone who dropped off the face of the earth! Pleasure to be back if only for a bit 🙂

I agree with the assessment that the wood slats are not original.

And I agree it is nice to see Kyle back.

Thank you very much to all for your comments.

I checked