Those are RCBS competition seating dies with bushings for different calibers. Their thimble is graduated and looks like the same type on the RCBS Precision Mic; their case headspace gauge. (Yes, Mr. O'Hier, cases do have headspace; just like rimless bottleneck chamber headspace gauges that they're calibrated and locked to zero on.)
Bushing hole diameters are a few thousandths larger than loaded round neck diameters. I've got a couple of them. My "30" ones are for .30-06, .308 Win, and 30 caliber magnums. A shellholder extension is used with cases about 2 inches or shorter in length so the primed and powdered case goes up into the seating chamber far enough.
Easy and fast to use, seats bullets straight if the case necks are sized straight.