“We do not have a formal serial numbering system. All of our stuff is hand built, so they all differ slightly. production dates can be found hand written inside the cabs of all of the amps and cabs, next to the builder’s signature.”

[Source: Josh, Electrosonic email 7/2/2007]

According to a discussion at the Gear Page in May 2010 Electrosonic amps are no longer in production.  They were made in Boonville, IN, USA

Examples of Electrosonic custom amplifiers from the internet archive

Electrosonic amplifier