Matt, There are three versions. Oldest are the bluish units Next the olive green units And last the silver unpainted units A few of the bluish ones do not have the TED installed and the lamp oscillator values are slightly different. The olive green units all have the TED The silver units have the TED but some have a coil shorting across the TED. See: https://www.eevblog.com/forum/metrology/the-hp5065a-and-its-ted-some-info !/ Cheers, Corby