Denotes specifically guitars, electric basses and amplifiers made by Fender Musical Instrument company from about 1946 until 1964. In 1965, company founder/owner Leo Fender retired for health reasons and sold his company to Columbia Broadcasting System. As CBS cared far more about profits than music, the cost-cutting it implemented at Fender resulted in the latter company's producing instruments which were almost universally considered far inferior after 1965 than before.
Thus, the term "pre-CBS" has come into usage, at least among musicians, to connote any product which is older and therefore better than equivalent items produced more recently. Its implication is similar to the the older expression "They don't make 'em like that anymore."
VIOLINIST: "My instrument is a 1729 Stradivarius."
GUITARIST: "Wow! So it's a pre-CBS, huh?"