I would love to see a detailed side-by-side comparison of the original Mlle. Sombreuil vs. the Sombreuil of commerce in the US. I know that it has been decided by authorities that Sombreuil is an impostor; but I think it would be useful to know who those authorities are, what criteria were used, and what systematic method was used, all documented in a published (online) work so that we might be able to refer to that work in order that newcomers to the idea might to attain the same level of conviction. I come to the table with a few disadvantages: 1) I've never lived in a place warm enough for teas to thrive. The only tea in my current zone 7b garden is Blumenschmidt, which almost does not grow here. 2) It's been over a decade since Sombreuil perished in my zone 6b garden and I have only recently planted it in my zone 7b garden, so a number of details about the rose are no longer fresh in my memory.

That said, I would like to suggest that it seems unlikely to me that Sombreuil is a first generation wichurana hybrid because:
1) All of the many first generation Barbier and Van Fleet wichurana crosses (that I'm familiar with) are completely cold hardy in zone 6. If I am not mistaken many are successfully grown in zone 5 and colder. Sombreuil, in my experience, is not reliably cold hardy in zone 6b. I'm not convinced it will grow in the summer coolness of my zone 7b garden. Its response to temperature seems identical to that of other tea roses I've attempted to grow and remarkably different to that of any wichurana hybrid rose I've ever grown.
2) Those same Barbier and VanFleet roses tended to have very long, whiplike canes and growth, which has not been my experience with Sombreuil. The young canes can be thin as are some young teas are, but they are nothing like as long. I grow Francoise Juranville and Paul Noel in my garden and even if none of the three were never to flower, it would not make sense to group Sombreuil with them.
3) Tea roses tend to have much fragrance, I am told. And there is a group of tea roses that has a subtle ivory coloration to its petals which I think is sometimes evident in Sombreuil. I understand that wichurana x tea crosses might have good fragrance, too. But I keep hearing people saying "Sombreuil is nothing like a tea rose," and these are two characteristics such a cross would inherit from teas if it were a wichurana hybrid. So in these two respects Sombreuil is very much like a tea rose.
4) Some tea-wichurana crosses made flowers with the very narrow thin petals we see in Francoise Juranville which can be attributed to its tea rose heritage through Mme Falcot. Much the same might be said of Paul Noel. On the other hand, some of the wichurana climbers descending from hybrid teas have wider petals. So, again the narrow petals are a distinctive tea rose trait. The point is that certain teas make distinctive, narrow petals and there is something of this quality to Sombreuil. Again, this point standing alone would suggest the possibility of a wichurana x tea cross; but
5) I cannot think of another wichurana x tea cross that has resulted in flowers quite so large as those of Sombreuil.

So its responses to cold and to coolish weather, its stout canes, the color of its flowers, the petal shapes in its flowers, the petal number in its flowers, the petal arrangement in its flowers, the hint of ivory in its flowers, and the fragrance - all of these things are suggestive of tea rose heritage, IMO. Response to cold weather and the nature of its canes make it very difficult to believe Sombreuil is a first or second generation wichurana hybrid.

Furthermore, when I make a side-by-side comparison of the leaves of Blumenschmidt (ARE) and Sombreuil (ARE) I find very little notable differences. After taking one each from my garden I find that had one had not been 1/4 of an inch larger than the other, I'd be very hard pressed to distinguish them from each other. They are the exactly the same color. They have the same level of gloss. They have exactly the same width. They have almost the same length. And they have the same size of serration at the edge.

There is one feature of Sombreuil that I am at a loss to explain: the flatness of its open flowers. Is that a tea rose characteristic? Or is it a trait unique to Sombreuil?

Of course, my impression that Sombreuil must be a tea rose comes in part from the fact that when I grew it, it was widely regarded to be a tea rose. Since it was the first tea rose I grew and I've never since been completely successful in growing a tea rose, it serves as my own mental prototype of a tea rose. It's a prejudice I fear I shall never completely escape.