[Keith Seymore]

"TBC" might be the engine broadcast code. They are usually a three letter combination, and are applied to the larger components (engine, trans, axles, radiators) that have a lot of part proliferation due to options. It's usually a paper tag applied to the front of the cylinder head or valve cover.

Rather than having the worker on the line try to comprehend all of the build variations and choose the right part, the engineers/assembly plant does the hard part for him. They might assign a TBC code to a certain engine; when the build manifest shows up for the particular truck it tells him to grab a TBC (as opposed to a TAC or TAB or some other engine) and bring it over to the line.

I'm guessing, btw, in the absence of any other information....

K