Ford just rolled off the 10 millionth Mustang, since 1964, so that 15 million number is astonishing for the Model T.
Evidently, the reverse gear had more torque then the forward gears and you would sometimes see them being driven uphill backwards. https://www.history.com/topics/model-t
I remember hearing that from my grandfather. He was an excellent machinist and had a huge, fully equipped machine shop in his basement. He machined and heat treated replacement rear axle shafts for his Model T and always carried spares when driving. They were notorious for breaking.
They were driven backwards uphill because they didn’t have a fuel pump, it was gravity fed. If you went up the normal way if the hill was steep enough you’d stall.