Gibson started making the LG series in 1942, starting with the LG-2. There were 4 in the series LG-0 to LG-3.
They introduced the LG-0 in 1958 and discontinued them in `74.

The serial number should give you an indication of what year the guitar was made. From `52 to `61, they used a letter, starting with Z in `52 and progressing backwards thru the alphabet to Q in 1961. After that, they went back to straight numbers again, starting with 100 in `61 and going up to over 972000 in `69. After that, in `70, they used a 6 digit number system again, and later the 6 number system with a two number prefix. I remember searching at the Gibson website and finding the serial number range for an old guitar last year. The info is there; just takes a bit of searching.

Yes, Gibson guitars suffered in the 70s and early 80s, due to problems with humidity control when they moved their plant from Kalamazoo, Mich. to Nashville. That was due to ill-advised management when the Norlin conglomerate bought out the Gibson company. Also around this time, they started building their guitars much much heavier (thicker tops, heavier bracing) to cut down on warranty work, As well, many of the skilled luthiers at the old Gibson plant chose not to stay with the company and relocate under Norlin management. Norlin seemed to figure their electric line was worth more attention at that time, I guess.

In 1985, a group of guitar enthusiast businessmen and luthiers bought out Norlin and in 1987, they purchased the Flatiron mandolin company in Bozeman, Montana. Their acoustic guitars are now built there, by very skilled craftsmen again and standards are back to what they were before the Norlin fiasco.

Anyway, those LG guitars are real beauts! I cut my teeth on my dad's old LG-1 (sadly, it's in pieces now, the headstock breaking on it, twice) and it's still my favourite guitar. The LG-1 was built as a bargain version of the LG-2, with simple ladder bracing and laminated sides, plus a few less fancy frills. I remember the top on it was bellied up quite a bit and the bridge had started lifting off and it needed a fret job, but it still played like a dream and sounded sweet as sugar. I'd sure like another Gibson LG guitar today, for songwriting, picking at home and recording, but I sure couldn't afford to buy one. Guess I'll take some measurements and have a bash at building one.   ;)

One piece of advice: Never put heavy strings on your LG-0. That was a major problem with those guitars way back when. They weren't designed for that kind of tension. Gibson always built as light as they could (thinner tops and light bracing, smaller bridges) which was what gave their guitars that "Gibson" sound that was so popular. A very different sound (and a superior one in my personal opinion) to the Martin sound. The Gibson factory saw a lot of their older guitars like the LGs coming back for repairs to tops, bridges and bracing due to the use of heavier strings by their owners. I wouldn't go any heavier than medium-lights on it to be safe.

(PS. I happen to be reading Gibson's Fabulous Flat-Top Guitars: An Illustrated History & Guide right now. That's where the info comes from)