Many thanks to you all for the appreciation - I am glad you agree with me that the tailor has earned it.
As far as I know, a Chestferfield may or may not have a velvet collar; I liked the self-cloth collar better as being more discrete and let the silhouette of the coat play the main part. I asked that the vent start below the seat because I thought it ballanced the length of the skirt better. The vent is offset from the backseam and is horizontal (not with a diagonal seam) and indeed "L"-shaped - I am not sure what it is correctly called in English; it is similar to the way a tailcoat's vent is done. I'll post a detailed picture.
I have added a picture taken from the side to cover all angles (excuse the yellowish light)
Here is the outside of the fly in more detail in case it is of interest:
My next project is a Guard's coat from the same tailor. I hope for an equally pleasing result and I look forward to reading your valued ideas on how it should / could be made in the thread I started in the Dressing Room.