I would go with your guess, Margalit says 18 - 19 mm is standard for
750 ml bottles and a #9 is standard for them. A natural # 9 is 24
+/-0.5 mm in diameter. Synthetic diameters vary due to the density difference of the material, a Guardian is narrower than a NomaCorq or SupremeCorq since it's denser. 17 seems a bit much to compress it.
Jellenek has some QC info on their website related to corks.
In my experience, it seems to depend more on the corker & the corks than the bottles. Although since I'm not a fan of hand corkers, I will GUESS that hand corkers MIGHT have a hard time with the 17mm openings.
The floor corkers that I have used (at least two different ones) have had no problems inserting #9's into a random selection of bottles. Sorry I can't guarantee any 17mm bottles cause I just got some cheap calipers, and I've been using them to measure corks rather than bottles.
Why measure corks? Because people with hand corkers seem to have problems with the #9's that are 24mm across, and don't have problems with the #9s that are 23mm across.
BTW, last week bottled a batch with a blue Ferrari (Italian) floor corker (brass jaws). Used 24mm #9s in a random selection of bottles. No problems.
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