We have recently introduced a optional base plate add-on for both the MF300 and MF500 fixtures. The picture shows one fitted to the MF500 fixture.
A base plate does several things. Most significantly it provides protection of the wiring under the probe plate from handling. That makes a fixture more suitable for use in a production environment where it might be subject to more careless handling than in the engineering lab. In fact, if you have a fixture that you have been using in engineering that is going to transition over to production use, a base plate is a great add-on.
Base plates also give the fixture a little more rigidity, which can be useful with a very large number of test probes, or if the fixture has to withstand other twisting or bending forces as a result of special connector arrangements, etc.
Base plates can also provide an extra surface to mount electronics within the fixture, which can be useful if the underside of the probe plate does not provide enough room. Base plates are made from the same acrylic material as the pressure plate and are just as easy to drill.