I just bought two new in the box 0A to 5A Simpson RF Ammeters on ebay. I asked the seller what the backsides looked like and he said they were metal. I then went to Simpson's website and fired off a few emails trying to find out if they had the thermocouples built in or not. The head engineer said we cannot help you because the meter was at least 15 years old. Yeah right, more like 50 years old I then finally received another reply from an old Sr. Tech. Plant Services & Product Repair guy at Simpson who remembers working on the meters years ago and he said they had internal welded cross thermocouples and were self-contained. The metal housings were probably for RF shielding. Anyway, I bought the last two the guy had, because you just cannot find these meters still new and unused anymore and with the 5A scales and I needed one for my HB antenna coupler. I have an old 3A Triplett in there now, but I don't yet have the thermocouple for it and 3A is really too small anyway. 500W carrier out into 50-ohms is 3.16 amps so the 3A meter would often get pegged. The 5A meter is perfect. I did find an old 10A Westinghouse meter a few years ago, but 10A is too big. It does have the brown back so it has the built-in thermocouple. That was the indicator on many of the old RF ammeters. If the backs were black then you needed an external thermocouple. If they were brown they were built-in. I guess Simpson started making them with metal housings for RF shielding with the built-in thermocouples so not all brands were the same.