Around 1993 Chrysler decided to change the transmission designations to something that described them better. So a 518 became a 46RH. It breaks down like this.

4 = number of forward speeds

6 = an arbitrary number for its torque capacity. A 2 is less than a 6 and 6 is less than a 7 and so on. It does not correlate to an actual number, it's just relative.

R = rear wheel drive and

H = hydraulically controlled shifting.

A 904 became a 32RH a 727 became a 36RH a 500 became a 42RH.

When they went to electronic control the H was replaced with the E so an electronically controlled 4 speed rear wheel drive transmission became a 46RE, 47RE and so on. I'm not sure if there even was a 42RE but I don't know why there wouldn't be as they used the same valve body and overdrive unit as the 46's.