I was replacing felt wheel seals in one of my T's the first of the week. I worked and worked attempting to get the felts in where they belong with no luck. I have did this many times, so it wasn't new to me. I then pulled out the remains of the felt that I was trying to get in and found a metal piece that came out with it. It was the remains of an aftermarket seal that was made from spring fingers, leather and a washer. I have learned that one must look inside the axle tube before installing felt seals. That will be 3 hours of my life that I will not get back...
Ah yes, the dear old leather seals. The knock on those is that sometimes those steel fingers cut into the axle shaft and cause leaking. The current neoprene seals are a big improvement. But they do need to be out of the way when you shove in the felts.
Good thing to get out. I won't tell you how I know!
I think those fingers not only cause leaks, but it seems to me there have been cases where they actually caused the axle to break from the groove causing a stress point. Makes sense to me. As of yet, I haven't had any experience with the neoprene seals in a T, but from the experience I've had with my TT, I wouldn't use anything else. JMHO. Dave