(Reuters) - Billionaire investor Mario Gabelli's investment firm is seeking more details about the valuation of National Amusements assets, the investor told Reuters on Friday, signaling the firm may challenge a landmark entertainment industry deal inked this week.

Skydance Media and Paramount Global ( PARAA ) agreed on Sunday to merge, in a deal that would entail Skydance acquiring the Redstone family's holding company, National Amusements, which owns a controlling stake in Paramount.

Gabelli Funds, which owns nearly 4.9 million Class-A voting shares in Paramount, is seeking greater transparency around the National Amusements deal, according to another source at Gabelli. It sent a letter to Paramount's general counsel on Friday, requesting information, the source said.

"Operation fish bowl ... most likely starts today," Gabelli, the 82-year old chief executive of Gabelli Asset Management Company Investors wrote on social media platform X.

Skydance Media and its deal partners plan to acquire National Amusements for $2.4 billion in cash. In a related transaction, the independent media company led by David Ellison would merge with Paramount in an all-stock deal that values Skydance at $4.75 billion, creating a company with an enterprise value of $28 billion.

The companies have declined to disclose how much money Shari Redstone would be paid for her Paramount shares compared to other shareholders.

Skydance's deal to buy Redstone's National Amusements values the holding company at $37.79 per Paramount share, and the Redstone family stands to pocket $27.55 per Paramount share because of $650 million in net debt.

This does not strip out some of the other assets that National Amusements holds besides its Paramount stake, including movie theaters and real estate. National operates 759 screens in the U.S., UK and Latin America.

The parties have declined to provide sufficient information for a direct comparison with the $23 that Class-A stockholders will receive for each share and the $15 Class-B stockholders will receive for each share.

National Amusements and Paramount did not immediately respond to requests for comment.

Gabelli has privately said for months that the deal could be harmful to investors and now appears to be laying the groundwork to possibly challenge it in court.

Earlier this year, the Employees Retirement System of Rhode Island, an investor in Paramount Global ( PARAA ), asked for documents related to talks between the company and Skydance.

Rhode Island argued that it wanted to be sure that the best price is negotiated for Paramount.