10/50 would make a strong but slow T - similar to the "high speed" TT optional worm gear with 5,17:1 and a recommended max speed of 22 mph.
Was it ever offered as an accessory? Haven't seen it in the books or old ads, but here is a pictured set:
Might be for another brand of car?
A 9 tooth to run with the 40 is available from Chaffin's. It gives you a 4.44-1
That ring gear looks too large to fit the Model T differential, just a guess as there is little get a true size in the photo.
It would make the car slow but low would pull up the side of a house!
I purchased the famous hill climber, La Rue Thomas's garage contents when he passed away. and there was a 10-50 set of gears in the stash. I sold it two years ago, sorry no pictures. I still have a bunch of speed equipment and four oil pressure racing engines.
Thanks, Frank for the confirmation. There seems to have been more stuff available for the T at one time or another than we can ever imagine