I know it goes with the HP41. What does this module do?
It provides functions for controlling the 3421 data acquisition unit. data Logging functions and the like. Let me know if you want to sell it...
See pictures at:
http://images.honesty.com/imagedata/9/9/1/16796991.jpg
http://images.honesty.com/imagedata/9/9/2/16796992.jpg