I don't believe Chevrolet made a factory tandem axle until 1956. There were many earlier tandems but they were aftermarket setups, so your '55 could be either AD or TF. There have been a few shown on Stovebolt and I'm sure you'll be hearing about them soon.
Often owners on these old trucks don't even have the year or sometimes even the decade right. I do hope its a good buy for you, keep us updated.
James
