UPDATE 3-Von Finck family sells chunk of SGS stake

* Von Finck family sells 40 pct of its 25 pct SGS stake

* Stake worth around $990 mln based on Tuesday close

* Sells stake to Goldman Sachs, Deutsche Bank for placing

* SGS declines comment on whether it will buy shares

* Shares down 6 pct

(Adds analyst comment)

By Katie Reid

ZURICH, Nov 11 (Reuters) - SGS (SGSN.VX), which quality checks goods from London's black cabs to toys, said its largest shareholder had almost halved its stake, prompting the inspection services company to reiterate its full-year guidance.

SGS gave no explanation on Wednesday as to why Germany's von Finck family had sold the 10 percent stake, worth around 1 billion Swiss francs ($992.1 million) based on Tuesday's closing share price, and its shares tumbled to a fresh three-month low.