It looks like this Part has been superseded by LEGO Part 6538b Technic Axle Connector Ridged [with x Hole x Orientation].
Click on a colored part below to see the Sets and MOCs that use part 6538a Technic Axle Connector Ridged [with + Hole + Orientation] in that color. You can find an overview of all LEGO Colors here.
If a color is missing, it probably needs to be added to a Set Inventory.
This part has been used by MOCs in the following colors but not officially released.
There are 3 Mold variations of part 6538a - Technic Axle Connector Ridged [with + Hole + Orientation] in the database, in use between 1993 to 1999. See Molds and other related Parts.