Thanks to John King's research, I obtained the correct dimension for the 3/8" nut that holds the spool to the driveshaft for 1910 cars. The hex is 5/8" like the later nuts, but threaded 3/8 by 16 tpi. It is 1/2 inch tall. Very unusual considering the internal thread for my 1910 RE housings is 3/8 by 24 tpi. That means the studs have two different threads on them, one for entering the RE, the other for accepting the nut at the other end. According to how I read Trent's research, some of the 1910 housings accepted a 3/8 by 24 inch stud and some accepted the 13/32 by 16 tpi type. All very strange.
You guys are sharp!
To the best of my knowledge they are all 24 TPI. They are a special bolt and may be addressed as 3/8+1/16 or 13/32. A standard bolt will fit but it is not correct and will be very loose. I have an old set of taps and dies that are stamped 3/8 + 1/16, I have heard of other sets that are stamped 13/32. They are a special oversize bolt.