[ by Melanie Gretchen ] Knight Capital Group's knights in shining armor – 6 firms that rescued the financial services firm after its software glitch earlier this month – have sold some of their preferred stock in the firm to Scottrade Inc, at Knight's behest.  According to 3 people familiar with the situation, who asked not to be named because they signed confidentiality agreements, Scottrade (one of Knight's biggest customers) will become one of its largest, long term  investors. Saving Knight. Knight's crisis commenced on the morning of 8/1/12 when a software glitch flooded the New York Stock Exchange with unintended orders for dozens of stocks. The error cost Knight, which sold most of its unwanted shares at a discount to Goldman Sachs Group, more than $400 million in losses. Following Knight's software glitch, the 6 invested $400 million into the company on 8/6/12 to prop up Knight which was teetering under the pressure from shocked investors.  To date, 2 of the firms, private equity giant Blackstone Group LP and high-speed market-maker Getco sold some of their securities, according to regulatory filings on Knight's website. In addition, at least 1 other firm is expected to make a regulatory filing about its sale imminently, said one of the sources, and the other 3 are expected to follow suit.  The securities pay a dividend of 2% and convert if the stock closes at $3 or higher for a fixed period of time, toward becoming one of its long-term investors. The Others. The 6 also include discount brokers TD Ameritrade Holding Corp, another one of Knight's largest clients.  Blackstone, Getco, TD Ameritrade, New York-based investment bank Jefferies Group, St. Louis-based brokerage firm Stifel Nicolaus and Arkansas-based broker Stephens Inc cumulatively bought preferred shares at $1.50 a share that are convertible into about 267 million common shares, or 73% of Knight. For further details, go to [Reuters, 8/22/12].