The 4 setscrews secure your tool bits. There are some marks on the milled flat where you tighten your tool holder screws against the flat.
I took some approximate quick measurements... They may be off a thousandths or two.Also the Kennametal brazed on carbide bit shown in the photos is NOT INCLUDED. This one would fit in a 2 boring bar holder or lathe tool holder. It uses 3/4 HSS or brazed on carbide tool bits. Bit is secured by up to 4 setscrews.
It is 20 long overall. Shaft is 15" long x 1.936" diameter. Tool holder head is 2-1/2" diameter x 4.725" long. The item "LARGE Lathe Boring Bar 2 Shank 2-1/2 Head 20 Long with 3/4 Bit Holder D412" is in sale since Thursday, March 26, 2020. This item is in the category "Business & Industrial\CNC, Metalworking & Manufacturing\Metalworking Equipment\Other Metalworking Equipment".The seller is "hollowpoints" and is located in Uhrichsville, Ohio. This item can be shipped to United States.