Several large companies that received letter bombs this week reported yesterday that they had earlier ignored extortion ‘letters demanding that they (place payments in bank accounts in Matamoros. Mexico. In one case, a $16 million certified check was demanded. Matamoros is just across th Rio Grande from Brownsville, Tex. As of last night, a total of 16 envelooes cdntaining lowgrade explosives had been delivered to offices around the country, including six here. No new letter bombs were reported by the Federal Bureau of Investigation yesterday. According to one company official, the F.B.I. has found that extortion demands sent list fall to 200 business executives and large companies generally called for payments to specified accounts in the Matamoros bank.