The resistor being measured is a 250uOhm 1% resistor manufactured by Stackpole. The datasheet is not clear about how the device measurement is performed, though the resistor element is specified to be within 1% or 2.5uOhms.
Both the OMICRON Lab Bode 100 and the Agilent E5061B are capable displaying impedance in the 2-port measurement. They support the OPEN, SHORT and LOAD calibration in the 2-port series and shunt measurements for improved accuracy.
A common mode transformer is necessary for all measurements where the ground braid resistance is significant, as shown in Figure 2, unless the VNA has a differential or semi-floating 50? input. The Keysight E5061B is unique in that its low frequency gain-phase ports have semi-floating inputs and do not need the transformer. The higher frequency Port 1/2 inputs, as well as the Bode 100 and most other VNAs for that matter, do require the transformer in order to make the 2-port shunt measurement. The J2102A gets well above 100MHz, which is sufficient for general PDN use. Above that frequency the transformer is not really impactful.