David L. Kugel, a former trader at Bernard Madoff Investment firm is expected to plead guilty on Monday to conspiring to defraud firm customers beginning in the early 1970s, two decades earlier than Mr. Madoff claims to have started his Ponzi scheme, according to a letter sent to Judge Laura Taylor Swain by federal prosecutors.  Madoff, who is serving a 150-year prison sentence, has consistently maintained that his enormous Ponzi scheme did not begin until 1992.  Prosecutors have disputed his version of events, and Mr. Kugel’s plea would be the first opportunity for them to present evidence in court that challenges Bernard Madoff’s claim. Given how small the Madoff firm was in the early 1970s, Mr. Kugel could be a significant witness for prosecutors as they continue their investigation. One subject of their inquiry is Peter B. Madoff, Mr. Madoff’s younger brother, according to court documents filed by Peter Madoff’s lawyers in response to civil litigation pending against him. Peter Madoff had worked at his brother’s side at the firm since at least 1970, when he completed law school. John R. Wing, a lawyer for Peter Madoff, said he had been unaware of Mr. Kugel’s apparent cooperation with the government and repeated that his client had no knowledge of the Madoff fraud, which cost thousands of investors $64.8 billion in paper losses and roughly $18 billion in cash. Kugel would be the fifth former Madoff executive or employee, including Mr. Madoff, to plead guilty to participating in the fraud. Daniel L. Zelenko, a lawyer for Mr. Kugel, did not immediately respond to a request for comment.   For further details, go to : [Dealbook 11/17/11]