1973 AMPEG V4-B
The "Blueline" silkscreened graphics changed to "Blackline" and were no longer etched into the metal.
Other 1973 V4B characteristics: Aluminum D shaft knobs, two input jacks, black rocker switches, three black metal power switches, two small round power lights, silver grille cloth with no badge logo, two prong power cord and outlet, fuse, white silkscreening, Magnavox manufacturing, serial number sticker on the back, impedance switch, two metal cord wraps, black painted chassis, black transformers, tubes, black capacitors, vinyl handle, black vinyl covering applied over black painted wooden box, four rubber feet, four chassis connector bolts, literature.
Ampeg serial number dating is undocumented and very difficult. Some examples of 1973 serial numbers are 2501x and 2914x. For more precise dating cross reference the serial number with the amplifier's transformer, potentiometer, capacitor and speaker codes.
Other codes to determine the manufacture date: The large power transformer should have a six digit code stamped into the metal. The first three digits indicate the company's personal manufacturing code. Those should be 682, which indicate the Electrical Windings Company. The fourth number indicates the year. The fifth and sixth numbers indicate which week of that year.
Potentiometers are also date encoded. There is a seven digit code stamped into the metal. Again, the first three digits indicate the company's personal manufacturing code. Those should be 137, which indicate Chicago Telephone Supply or CTS. The fourth and fifth numbers indicate the year. The sixth and seventh numbers indicate which week of that year. Capacitors and speakers can also be dated in a similar manner.