The rear derailleur tension adjustment screw on my 1997 or 98 Veloce 9speed bike is broken. The correct replacement part seems to be the Rd-At115 screw.
However, there seem to be at least two more such screws with part numbers RD-DA004 and Rd-Sr014.
Here's my question: What is the technical difference between these three screws (that seem to do the same thing)? Diameter (outer? inner?), steepness of the tread? Something else I've overlooked?
Also, if they are indeed different, what would be the reason to change the specification of this rather trivial part?