The rear toolholder has been an invaluable addition to the Hardinge. It allows "finishing" tools like cutoff blades or beveling tools to be installed and used without changing the setup on the front toolholder. The only thing different about them is that they generally need to be installed upside down, due to the rotation of the spindle. This is actually a tremendous help - especially with tools that have a long overhang - because the geometry tends to retract the tool tip whenever it tries to "dig in", making parting off much more stable.
This particular set is a "stacked" design, with an L4 intermediate spacer block and an L3 precision sliding tool block on top of it, permitting extremely fine adjustments of the cutting tool at the rear that are independent of the cross slide. This allows quick alignment of a tool in the front toolpost and the one in the rear for simultaneous machining of a part.
Brown and Sharpe screw machine style toolholders fit the slot, providing a wide range of special cutting capabilities. This particular cutoff toolholder is actually an old Luers (Empire Tool Co.) product, (like the "official" L10 Hardinge HLV-H rear cutoff tool holder), but is more massive. When it comes to a toolholder, massive is better... The tool is an original Empire 1/8" cutoff tool. The workpiece in the collet is a change gear retaining bolt like the one sitting on top of the compound, missing from my lathe. I decided to make one (out of 304 stainless because the lathe machines it so beautifully) after I got a quote from Iverson in the amount of $96 for one. Two hours later, it's finished.
(Right click the photo below to enlarge)