You’re right not to hold your breath, especially since there is no such thing as “original Lillet Blanc”.
There’s an interesting doctoral thesis on Lillet written in the mid-80’s by a French historian specialised in small, family-owned local businesses. He had access to the archive and the correspondence of the family, and it shows that Lillet was constantly evolving. There’s no original formula, but a series of changes and adaptations. The product also varied according to where it was sold – and to whom.
When Lillet was launched, it was Sauternes-based and fortified with Armagnac eau-de-vie. A ‘double quinquina’ version was sent to the colonies. When it finally found some success in the UK, Sauternes and Armagnac were distant memories – and the market got a special, drier version. The Lillet that made it big in the US in the 60’s went through a couple of different formulas. And so on and so forth.
So on what formula do we settle? As cocktail people, I would assume we’d say “the English one, the one that made it into the Savoy Cocktail Book”. That would be the Lillet ‘goût anglais’, drier, stronger (18°) and, according to the family’s correspondence, more bitter. Thing is: as far as we know, the formula hasn’t survived and we’re not even sure this is the one Craddock, Tarling and others were actually using – or rather what version they mean when they mention the brand.
In any case, odds are that the ‘goût anglais’ didn’t taste anything like Cocchi Americano or French quinquinas such as Cap Corse or new brands based on old recipes such as Aéro d’Or. Americano is vermouth with extra gentian - quinquina is a sideshow. It can - and should - be excluded as an ‘historical’ alternative. Ditto French quinquinas: they are mistelle based and heavy on the aromatics. Lillet never was. From early on, Lillet was developed as a product with strong wine character and discreet aromatisation. Its selling point compared to other quinquinas was the quality of the wine and the elegance and lightness of the formula. If us ‘authenticity’ obsessed cocktail freaks had been around back in 1895, we might have just been treating it as we treat some so-called American ‘vermouths’ today… “Not a real quinquina, bro!”
For what it’s worth, I tasted a couple of samples of French 1960’s Lillet and their bitterness while more marked than today’s product, was discreet. If I remember correctly, @Splificator once said that the samples he got access to were not bitter at all. But I don’t think either us got to taste something pre-war or ‘Goût anglais’.