Origin of "Best boy" - a film crew position

The term shows up in credits as "Best Boy Electric" or "Best Boy Grip." Imdb explains the origin of the term thus:

The origin of the term is from "pre-union" filming days when the line between Grip [department dealing with cameras and other equipment] and Electric departments was less rigid. When the head of either department needed another body temporarily, he'd go to the head of the other department and ask him to "lend me your BEST boy". By default the 2nd in charge of either department came to be known as best-boy.

This exact wording appears in multiple places across the internet, sometimes with attribution to IMdb and sometimes without. Imdb gives no source for its explanation, which sounds like folk etymology to me.

Long before the film industry came about, English public schools had the office of "best boy," or teacher's assistant, described here in an 1892 issue of The Public School Journal:

Not only can the best boy in school light cigarettes for his teacher, but he becomes the monitor of the school room when the teacher is hearing recitations.... [H]e patrols the aisles of the school room, ... calling the other boys to order....

The term was also used on sailing ships for the captain's apprentice as recorded in the journal The Nineteenth Century and After in a 1921 article called "Sea Service" which reported on an apprentice's drowning:

The captain was almost overcome by the loss of 'his best boy.'

You can find claims that early movie crews were made up longshoremen and sailors. (See this Forbes article, for instance.) Again, no sources are cited.

There is only one Best Boy and he is in the Electrical dept, the use of best boy electric has only crept into the game since the 80’s. The term is associated with the term Gaffer, therefore Best Boy Grip needs the suffix Grip. The origins are scattered, from Gaffers who rigged sails on tall ships putting his Best Boy on each spar, One Gafffs sails, and a lot of early film lighting involves rigging ( or Gaffing) sailcloths and other overhead textiles to control lighting, this predates film eclectic lighting and is everyday work today. There is also reference to the potato famine of Ireland whereby Gaffers would bring work gangs to England to pick crops, promoting a Best Boy to manage each workgang, usually per field or task. Then there are the myriad of other uses whereby a ‘Boy’ was promoted to crew chief. The suggestion that one calls for another dept Best Boy is bunk, the Best Boy is the last member to be released to another dept, he is after all that departments cheif asset, least not in my 35 years as a Best Boy. Cheers

Jonathon Green, Dictionary of Jargon (1987) has this entry for best boy:

best boy n. {Film} a film crew member who is the assistant to the gaffer [chief electrician] or key grip [supervisor of employee who move around various specific types of heavy equipment].

But this only tells us what the best boy does and puts the origin of the term at no later than 1987. The Internet Movie Database site has a somewhat more helpful glossary entry for the term:

Best Boy

AKA: Assistant Chief Lighting Technician, Best Boy Grip, Best Boy Electric

The chief assistant, usually of the gaffer or key grip. In charge of the people and equipment, scheduling the required quantities for each day's work. The term originates from promoting the crew's 'best boy' to supervising, allowing the gaffer and key grip to stay on set and carry out the cameraman's lighting needs. The origin of the term is from "pre-union" filming days when the line between Grip and Electric departments was less rigid. When the head of either department needed another body temporarily, he'd go to the head of the other department and ask him to "lend me your BEST boy". By default the 2nd in charge of either department came to be known as best-boy. This term may also have been borrowed from early sailing and whaling crews, as sailors were often employed to set up and work rigging in theatres. There are no "best girls" per se; female chief assistants are also called "Best Boys".

According to the Infoplease entry for "Movies and Films: Unions" unionization of actors, cameramen and other U.S. film industry employees occurred began during the 1910s and 1920s. However, Merriam-Webster's Eleventh Collegiate Dictionary (2003) reports a first occurrence date of 1937 for best boy in the film industry sense of the term.