Former Riggs National Bank Chairman Lewellyan Jennings and Steuart Investment Co. President Curtis Steuart will step down from the bank's board of directors next month -- paving the way for election to the 25-member board of two nominees selected by Dynalectron Corp. Chairman Jorge Carnicero, who recently became the bank's largest stockholder.

A proxy statement mailed yesterday to shareholders of the area's largest financial institution also disclosed new information about FinAmerica Corp., a Carnicero-controlled investment firm in Delaware that owns most of his 10 percent interest in Riggs.

According to the proxy, mailed with the Riggs annual report in advance of the March 14 stockholders' meeting, minority owners of FinAmerica Corp. are his wife, Jacqueline Carnicero, and Arbol Solo, S.A., a Buenos Aires company.

Arbol Solo was described as a "large corporation" engaged principally in banking, cattle ranching and manufacturing, "privately owned by Argentinian citizens."

The statement did not elaborate on what type of banking business is operated by Arbol Solo nor did it state what share of FinAmerica is owned by the Argentine firm. Carnicero controls more than half the stock of FinAmerica through Trans-American Aeronautical Corp., another Delaware firm engaged in financing overseas airplanes, hotels, and construction.

Of Trans-American stock, Carnicero owns more than 90 percent while four unrelated and unidentified individuals own a minority interest.

Jennings, who served as Riggs chairman and chief executive from 1963 until his retirement in 1973, has been a director of the banking firm since 1963. Steuart, whose investment firm is an umbrella organization for diverse petroleum and auto dealer interests in the area, has been on the Riggs board since 1968.

Both Jennings and Steuart are not candidates for election to the board because of a mandatory age limit for directors (72) and their departure opens up two seats for Carnicero's nominees -- daughter Jacqueline Carnicero Duchange and Dynalectron officer Merlon F. Richards.

The proxy states that Carnicero informed management of his intention to make the nominations and noted that he has enough shares to elect three directors. He has agreed to turn in irrevocable proxies for the election of himself and his two nominees and that he will make no additional nominations to the board at the March meeting.

Riggs Chairman Vincent C. Burke Jr. said in an interview yesterday that Carnicero "has been and is a good director." Asked if management had any concern about a possible takeover through additional Carnicero stock purchases, Burke said, "none whatsoever."

The proxy statement said Carnicero is continuing to buy additional shares (he owned or was related to ownership of 255,486 shares on Jan. 23) for the "purpose of investment only" but that Carnicero is "not in a position" to say whether he would elect additional directors in the future to reflect new holdings.

In the annual report, Riggs officers noted that loan volume last year soared 24 percent to $1.15 billion at year's end. Commercial loan volume grew by 17 percent to an average of $442 million in 1978 and loans made through a Nassau office accounted for 23 percent of the volume.

Burke said Riggs has a couple of loans outstanding in troubled Iran, but nothing of a significant nature. Non-performing assets of the bank (reduced interest loans, non-paying loans, other real estate) were reduced by 19 percent in 1978 to $13.8 million.

The proxy statement also revealed that a $9.3 million loan to director Karl W. Corby in 1972 -- later restructured to a balance of $2 million last year -- was paid in full on Jan. 17.