While digging up parts for Damnesia I took some pics for reference.
Here's the 3 stages of evolution I found to be distinctive. The early 1899 spindle was 1 piece with a thin shaft. The end of the rotor is longer and mates with the receiver. Later the rotor is shortened and the spindle becomes 2 piece with the head of the spindle shaft becoming the mating surface into the receiver.
The first 2 piece spindle on the left is different from later versions. Note the inside of the rotor heads. The lock nut is also different.
Closer view of the rotor heads.
These are the 2 spindle heads of the later concave rotor head style. Note on the left there is a hole for the spring tip. This head is thin and not threaded. The next version is much thicker and threaded for the spindle shaft. the spring tip is not visible.
A close view of the early 1899 spindle head. These are lots of fun to adjust. That is one of the pot metal rotors that show up sporadically.