Wima MKP10 (old stock): is it possible they've been polyester in the past?

Yes polyester. mrjayviper, I will look this morning for the caps.

In fact I just did. Ero MKC 6.8 µF 100V (2 pieces) and 1.5 µF MKP10 250V (4 pieces and you can use 330 nF in parallel to a pair). If it makes you happy you can have them for shipping costs. Since your country is listed shipping costs will be high.
 
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MKT/MKS = PET (polyester/mylar etc). MKT = Metallisierter Kunststoff polyesTer called MKS at Wima.
MKC = polycarbonate
MKP = polypropylene

MKC was quite good but slightly hygroscopic that is why these are usually molded in resin. When the single manufacturer of the capacitor grade polycarbonate foil/film stopped production no one could make capacitors with it anymore. MKC is better than MKS but not as good as MKP.

Then we had/have Styroflex/Polystyrol (PS), MKM, MKH, FKP, FKC, PPS, PTFE....
 
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