Inside micrometer screw gauge, more specifically a digitally read hole micrometer – often called a “Digimatic Holtest” or just “Holtest”.
It is used to measure internal diameters with very high precision, for example:
• Hole
• Drilling
• Cylindrical fits
Features:
• It has three expanding measuring arms (inside the head) that expand evenly as you turn the knob.
• There is a micrometer scale on the dial for reading, and there are models with a digital display (however, this appears to be an analog/digital hybrid).
• It has a millimeter setting ring and an adjustment screw at the back for accuracy.