The English Wikipedia explains the coding in the 'overview' section. In fact 100G reffers to a standard sedan (same body as the one which appears on the wikipedia links thumbnails). I doubt if it had vinyl roof as option, as it was used in more luxurious models. Could in theory be the 'Special' model, but apparently, according to Wikipedia, those had 100GB in the engine code (and probably also the VIN).
I've also read that the 850 Spider had a fabric roof as standard, so if this one shown on the Polish wikipedia is made from vinyl, it had to be made later.
I had both vinyl and fabric roofs on my cabrios (other than Fiats). Both solutions have cons and pros. But the 25 years old vinyl roof was so delicate that it could crack all over every second. I can't even imagine more that 50 years old vinyl roof, unless kept in a garage, away from the sun... When thinking on restoring it I spoke to several companies that were producing cabrio roofs and none of them had the possibility to make the roof from vinyl, everyone was using fabric instead. If properly taken care of, those are more resistant to UV and heat.
I also know that in the US vinyl roofs were very popular in the 70's and 80's
So it's hard to say, unless someone who knows the 850's history better can confirm.