I just want to mention again that all the issues raised with regard to module extenders are important,
and no matter which route you go, the result is always a bit sub-optimal, and awkward on the bench
The magic answer is just to take a TM502A apart, removing the blue outer shell. What's left is a compact dual connector power supply (complete with power switch + line connector), and total access to the module under test. This is the simplest, easiest, and most trouble free way to work on a TM500 plug in, and takes up the least possible bench space. I highly recommend this path over all others. A similar trick is possible with a TM5003 for the TM5000 modules, but it's certainly bigger.
I should point out that I did not move to this approach until long after I already had many extenders, and it's only this specific frame that yields such good results. In any case, it is a magical fix, full of test bench happiness.
Test plug-ins to validate the frame are still needed, and you may still want to extend a plug in from a specific frame, so that's the best case for an extender.
All the best,
walter (walter2 -at- sphere.bc.ca)