If a Beaugnier, they were hand made saxes. Then this Paris, France suburb factory closed its doors around 1970, because they could not compete with machine manufactured saxes.
I have their bari from the early 1950's, a Vito. The Vito was Leblanc's entry level instrument. My bari has the same left hand pinky keys as your tenor. I bought it for a song on an Internet auction 12 years ago. It was an ex-school instrument, well battered, taken out of circulation after many repairs, then stored for a while. Case was very musty, trashed it in short order and bought another.
It had less than 50% of its lacquer left, played badly out of tune, but had decent pads. I removed dents, straightened the key racks, resoldered separated posts, installed missing bumper corks, adjusted key heights, removed the red brass tarnish. It cleaned up nicely. (A slightly softer red brass was used, as it was easier to work by hand.) When through, found it easy blowing, very easy to hit falsetto, plays very well in tune. Has a darker sound than newer instruments.
Then, difference between a pro model and beginner was the amount of tuning, more engravements and embellishments, quality of jewels, and etc. But it has the same metallurgy. Although a Vito, now it plays like a pro model.
Time has passed, now I need to replace a few pads from my wear and tear. I wouldn't trade your Mark IV for it, but if you want an extra horn and love the way it plays, then go for it.