In a signal that informal talks to end the writers strike may be gaining momentum, the WGA has offered an olive branch to the congloms by abruptly canceling a Wall Street confab for CBS’ institutional investors.

The guild — now in its 87th day of striking — pulled the plug Wednesday without explanation on what would have been a presentation aimed at persuading investors to put pressure on CBS honcho Leslie Moonves to make a deal with the WGA.

Next Tuesday’s event at the Cornell Club in Gotham would have also been designed to persuade research analysts to lower their investment ratings on CBS stock. The get-together was billed as an hourlong event to present the WGA’s analysis of the strike’s impact on the congloms generally and CBS specifically, featuring speeches by WGA West president Patric Verrone, WGA East prexy Michael Winship, SAG president Alan Rosenberg and writers and actors from CBS programs.

Neither side had any comment Wednesday about the confab or the informal talks, which have entered their second week under a news blackout with the goal of setting the stage for the resumption of official negotiations.

But the WGA’s move to deep-six an event that could have angered the congloms will likely be interpreted by the town as a sign that the talks — despite their slow pace — are yielding some progress. WGA West exec director David Young noted in the invitation, sent out Sunday to research analysts, that most analysts view CBS as being “especially vulnerable” to the strike because of its concentration in network TV.

“An extended strike will affect the pilot season for fall television series, the customary upfront ad purchasing season in early May, the ability of networks to meet the required ratings performance for ad revenue already received, and important ancillary revenues on the sale of series to secondary markets,” Young said. “We will also discuss concerns regarding possible conflicts of interest in the governance of CBS given its relationship to Viacom and the different positioning of the two companies vis-a-vis digital media rights.”

CBS stock closed at $27.67 on Nov. 2, the last session before the strike started, and has declined 9% since then. It closed Wednesday at $25.03.

In another development Wednesday, the WGA inked its 14th interim deal by signing with year-old Overture Films on the heels of similar agreements signed last week with Lionsgate, Marvel and RKO. The interim deals let the companies hire writers under the terms and conditions offered by the WGA before formal negotiations with the AMPTP collapsed last month.

For the WGA, the deals are a way to gain leverage in persuading the congloms to resume formal talks.

Overture, backed by John Malone’s Liberty Media, launched operations in late 2006 headed by industry vets Chris McGurk and Danny Rosett. Its first feature, caper comedy “Mad Money,” opened earlier this month.

Rosett, whose title is chief operating officer, said in an interview that the interim agreement will allow Overture to continue begin moving forward on its 2009 slate. And he noted that as word got out Wednesday, the volume of inquiries accelerated.

“You could feel the momentum picking up this morning,” he added. “We feel like this is something that helps get us beyond being ‘in theory.’ We felt that we had a very reasonable conversation with WGA about what’s best for our company.”

Overture’s going into production Feb. 13 on “Humboldt Park,” which has a completed script. Upcoming releases include “Sleepwalking,” starring Charlize Theron, and Tom McCarthy’s “The Visitor.”

The AMPTP dismissed the WGA’s interim deals, labeling them as “one-off agreements” that are meaningless because the companies signing them know they will not have to abide by their terms for very long, since they’ll be superseded by whatever final industrywide accords are reached.

The AMPTP also joined the brawl over interpreting terms of the DGA tentative deal, signed two weeks ago. A day after SAG expressed its deep misgivings about the terms of the directors’ pact — drawing a rebuke from the DGA — the AMPTP took SAG to task Wednesday over its characterization of the compensation for paid downloads.

SAG had claimed that the DGA’s doubling of the download residuals formulas — to 0.7% for TV and 0.65% in features — was actually an AMPTP rollback since the WGA, SAG and the DGA had filed grievances over the download rate lagging the pay TV rate of 1.2%.

But the AMPTP said SAG’s wrong, since none of the guilds ever pursued the grievances; instead, they opted to hold them open and address the issue at the bargaining table.

“The bottom line is clear: The 1.2% figure was never a benchmark of any kind,” the AMPTP said. “It was merely a demand that none of the guilds ever seriously pursued.”

The AMPTP companies have maintained that electronic sell-through is another form of homevideo rather than pay TV because the buyer retains a permanent copy.

“If the guilds believed that they had a strong case, they would have pursued their grievance claims; instead, they held them open, preferring to address the issue in negotiations,” the AMPTP said. “This is what the DGA has done.”