Lego RCX 1.0 brick relationship to Robotics Invention System (RIS) & RCX firmware versions
The main differences between the versions were:


Firmware - although later versions of the firmware were backwards compatible with the earlier bricks
Power Adaptors:


RIS/RCX 1.0 had a power socket on the brick as well as battery options
RIS/RCX 1.5 removed the power socket from the brick
RIS/RCX 2.0 Functionally equivalent to RCX 1.5

IR Tower connections:


RIS 1.0 and 1.5 had a serial connection for the IR Tower
RIS 2.0 had a USB connection for the IR Tower, which doesn't operate nicely with 64bit Operating Systems (ironically* the serial connection does work with a serial-USB adaptor apparently)

Manuals - I believe the Constructopedia was more comprehensive in 1.0 than in 2.0.


* I say ironically as I originally returned my 1.0 or 1.5 set as I was unable to get the serial connection to work on my PC at the time and later bought a 2.0 which worked fine then, but not any more on my more modern hardware.