Are they galvanised cappings on the tub?
It's a matter of personal preference I suppose, but I would go with standard sills and avoid chequer plate.
 
You can get zinc rich paint. Not the same as hot dipped galvanised though.
 
Check it carefully and if there is no sign of corrosion under it or coming through, then it's doing it's job. Leave it and paint over it.
 
My td5 90 is the same as the OP picture. Mine are missing as it's got sliders there instead
 

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