Yet another specific gridfinity part.  This is a 1x2x6 (42mm tall) holder with orientation feature for a dial indicator.  The white part is not in the model.  It's a spindle adapter for the indicator.

Note that if you're a gridfinity purist, you may not appreciate that the attachment knob hangs out the back.  It's OK for me since there's room at the back.  It could be fully contained in a grid in a 2x2 version.  Comment if you'd like to see that version made.