comes up every so often, the year of an FC is what the owner sees on a title,
the problem is that leftovers that didn't sell, had the year that they were
sold, as the new year that they were registered.
But as a general rule all 56-early 58 were typically 6V, all later were 12V.
The problem or exception, is that some early 1958 pickup bodied with prefix 65548 already had 12V if a lot of them were sold in a big city, but the same vehicle made in 1958 could have had a 6V and was not sold in a small town until 1960, making that one a 1960 with a 6V system. I used to think that a serial number was also a way of knowing, but that is assuming none were ever wrecked and a complete cab or frame replacement didn't happen.
The FC-150 showed up in late 1956, some, a few, made in 1958 narrow tracks were 12V, for sure if it is a wide track it is a 12V.
The FC-170 came out in Mid 1957, some or few were 6V, by 1958 they were all 12V, again for sure if you have an FC-170 that has an improved cooling tunnel, it is a 12V.
Suggestion, if you are not doing an original authentic restoration, and you already have a 12V system in an FC, then leave it that way, don't replace all your electricals to 6V to be correct to a specific year or serial number. Jeep engineers really did make a great improvement when they went to 12V, from 6V.