One other thing to be aware of is that the early 039 serial number units have 165 VA output per handle whereas the later units have 180 VA per handle. Another factor that would favor a later unit. Having said that I have two of the first run and one later, and not one of them has caused any major issue. One toasted its cooling fan, which is super easy to replace, and its a generic computer cooling fan. Another toasted its adjustable pot which controls the voltage output. Again a very easy fix with a generic jobber pot. Otherwise rock solid. I will run them without a worry until they (or I) fail, then worry about replacements.
Rod