Gauge blocks (GBs) are very important material standards that provide industry with reliable and traceable standards of length. At the highest accuracy level, the measurement of GBs must be performed by interferometry. The double ended interferometer (DEI) offers an alternative to obtain traceable measurements of the absolute length of GB shaped objects without the need for a platen to be wrung to one of the faces. In spite of this general advantage, there is no reliable DEI system being used at any National Metrology Institute (NMI). PTB, as the NMI of Germany, is developing a dedicated DEI which will be situated in a temperature-stabilized vacuum chamber. This paper describes a preliminary prototype DEI that was built firstly to study the measurement principle, and secondly to learn about the challenges and limitations to be taken into consideration for the final design. A comparison of results from the DEI measurements and those from PTB's existing single ended GB interferometers (SEI) indicates that the present prototype is a basis for the final version.