(Dow Jones) Financial firms are wishing they had more time to prepare their trading systems for a new restriction on short selling that will hit the market in just seven weeks.

The Securities and Exchange Commission approved the curb on short-selling, referred to as the modified uptick rule, in February. But the industry is still awaiting the regulator's release of "frequently asked questions." The release is expected to provide answers to key questions about some of the specific ways the curb will be interpreted.

The restriction goes into effect Nov. 10. As that date gets closer, trading firms say, they are getting anxious about the looming deadline because they don't have all the information they need.

"It's been a little light in terms of industrywide guidance. There are definitely some unanswered questions," said Jamie Selway, managing director at Investment Technology Group. "It certainly would be helpful for there to be more time. At this point it seems like we're on the cusp. People are rushed and there's a lot of things in contention."

Under the new restriction, if a stock falls at least 10% in a single day, short selling will only be allowed on that day and the following day if the price of the sale is above the highest bid nationally. Traders short a stock by borrowing shares and selling them in the hopes of later repurchasing the stock at a lower price to make a profit.

The rule means short sellers, who benefit from declines in stock prices, would be blocked from dumping the shares at a cut-rate price during the day of the stock's slide, as well as the next trading day.

An SEC spokesman said "we have been in frequent contact with market participants as we continue to draft the FAQs, and are not aware of any impediments to the implementation of the new rule by Nov. 10."

For trading firms, preparation for the modified uptick rule includes making changes to the coding and programming of their algorithms, compliance reporting, smart order routing and exchange connectivity.

Leonard Amoruso, senior managing director and general counsel at Knight Capital Group, said his firm is preparing its technology as much as possible for the modified uptick rule, but that until it receives further details from the SEC, the firm can only do so much.

Not having the FAQ yet "certainly has a lot of firms concerned because nothing is ever easy," Amoruso said. "There's really no such thing as a quick fix anymore."

The unresolved issues include questions over how to label trades in a number of situations, including when a broker-dealer is long on a stock but acting on the behalf of someone shorting it.

Another open question is how to treat short-sale orders exempt from the rules. For example, orders can be exempt if placed by official market makers. Under the new rule, these orders labeled as "short exempt" would only be acceptable following a 10% drop putting the rule in effect, and traders would have to label the same orders "short" in normal conditions.

"Obviously that makes it difficult from a programming perspective," said Dmitri Galinov, director of advanced execution services at Credit Suisse.

The new curb is a variation of the former uptick rule, eliminated in 2007, under which investors could short a stock only after it had risen, or ticked higher. In 2008, the SEC placed an emergency order temporarily barring short-selling in 799 financial company stocks after Morgan Stanley's chief executive at the time, John Mack, along with former Lehman Brothers chief executive Richard Fuld and other heads of embattled financials blamed short-selling for their companies' sliding stock prices. Market watchers have long debated whether the rule's removal played a role in the market's swoon in 2008 and early 2009.

But traders said the fact that a similar rule previously existed doesn't help prepare their systems for the new curb.

"What appears to be on the surface a relatively simple change, given the highly interconnected and complex nature of our markets, turns out to be actually quite a lot of work to get done," said a person at a large New York bank. "We're doing not much else around our trading system other than getting ready for this rule."

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