Wouldn't exactly say "Another company took over production", it implies a change of personnel and/or location which didn't actually happen although it's an interetsing observation about the label change maybe coinciding with a new owner.
Guild built 'em at Westerly from '67 through '01 although it's true the company itself underwent changes of ownership during that time.
They were bought by Avnet Corp in '66, the rest is best recapped in this excerpt from the BLuebook of Guitar values:
"In mid August of 1986, Avnet sold Guild to a management/investment group from New England and Tennessee. Officers of the newly formed Guild Music Corporation included company President Jerre R. Haskew (previously Chief Executive Officer and President of the Commerce Union Bank of Chattanooga Tennessee), Executive Vice President of Plant and Operations George A. Hammerstrom, and Executive Vice President of Product Development and Artist Relations George Gruhn (Gruhn later left the company in early 1988).
Unfortunately, the remaining members of the investment group defaulted on bank obligations in November of 1988, leading to a court supervised financial restructuring. The Faas Corporation of New Berlin, Wisconsin (now U.S. Musical Corporation) bought Guild in January 1989. Solid body guitar production was discontinued in favor of acoustic and acoustic-electric production (a company strength) although some electric models were reissued in the mid-1990s.
Guild was purchased by FMIC in 1995. Guild introduced more solid body electrics and models were based on memorable Guild models from earlier years (i.e. Starfire, X Series, etc). In 1997, Guild opened a new Custom Shop in Nashville, Tennessee. In 2001 production was moved to Corona, CA. By 2006,"
So the label changes don't seem to coincide with any ownership changes, but it was a good question.
Fender finally sold the brand and its intellectual properties and even a fair amount of production equipment to current owners Cordoba Music Group in 2014.
Think Neal's correct on this as "early Westerly", I know it's covered in Hans' book (The Guild Guitar Book) which I don't have handy, but pretty sure once they started saying "Westerly" on the label it said it until production was moved to Corona, CA, and Tacomas say Tacoma, and New Hartfords say New Hartford.
In fact some early Westerly guitars still had "Hoboken" labels until '70.
Your s/n does jibe perfectly as an '81 according to their records, but I think a "Made in USA" only label preceded yours, not followed.