With the UT55A, UT75A, and UP55A, is there a difference between selecting RS485 communication with the 2nd digit of thesuffix code versus the 3rd digit?

Yes. The communication protocol is the same, but note the difference below. You can also install two systems for use with separate protocols.
The maximum speed of RS485 communication using the 2nd digit of the specification code is 19.2 kbps. The UT55A with the /LP option (power supply for 24 V sensor) and the UT75A are 2-wire systems. The maximum speed of RS485 communication using the 3rd digit of the specification code is 38.4 kbps. You can use both 2-wire or 4-wire systems.