How old is my guild x-50b serial#14388

The 6 digit number system was used from 1974 to 1979. Your guitar is a 1979. Depending on condition, it would sell for about US$2000.
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Normally it's on the back of the headstock on vintage Guild guitars

Guitar Info Your guitar was made at the
Corona Plant (Fender), USA
in the Year(s): 2009 - 2010


Fender: Corona Corona factory was opened in 1985, Corona California, with major guitar production starting in 1987. With the addition of a custom shop in 1987 the plant currently employs about 700 workers. Apart from general and Custom Shop Fender instruments and amplifiers, it also encompasses Guild acoustic & electric guitars.

1979

Your guitar was made in 1973. Without more information as to the model and condition, i can't offer an opinion on the value.

I can't find that one in the Guild website.

Can you tell me the model number of the instrument?

Because according to the Guild database, it doesn't exist.

Is it possibly one of those guitars "by Guild" like the Madeira, or the DeArmond?

For model years 1970 to 1979, the only information available is the first and last serial number produced- no model names or numbers are available. 1973 serial numbers are 75603 to 95496, so your guitar serial number would be towards the end of 1973 in Westerly, RI.

The value is hard to say- the instruments from that era can be valuable: instruments of that era have sold on EBay from between $800.00 to over $2,000.

As always, it's worth exactly what someone will pay for it- Good luck!

Guild started making the GAD series in China in the early 2000's.
The earliest mention I can find of the GAD D25 is 2007.
Hope this helps!

Try this link:http://www.guildguitars.com/support/contact_us.php