A glimpse backstage at contra-Sandinista peace talks. OLD FOES COMPARE NOTES

From behind tight security, and even tighter lips, word of what is happening in the cease-fire talks here between the Sandinistas and contras is beginning to leak out. The picture that emerges from two sources familiar with what transpires in the negotiations is one of forthrightness, and surprising harmony.

Indeed, at press time the talks were expected to achieve some sort of stop to the war. What was unclear was whether the two sides would agree to a definitive cease-fire, as the Sandinistas want, or a temporary truce. The Sandinistas have called for a 90-day cease-fire. The contras want a 45-day truce that would be renewed later, provided the Sandinistas made sufficient progress on the kinds of internal liberalizations demanded by the contras.

The talks themselves have provided interesting glimpses into the contra and Sandinista perspectives on each other and the country they have fought over for the past seven years. The two sides have never before held face-to-face negotiations.

On Tuesday, for example, Gen. Humberto Ortega Saavedra, commander of the Sandinista Army and brother of the President, listened while two contra comandantes, ``Fernando'' and ``Tono,'' related why they had taken up arms against the revolution.

Both were members of former dictator Anastasio Somoza's notoriously brutal National Guard. They told General Ortega of how they were outraged to see former comrades sent to prison after the 1979 revolution overthrew Somoza.

Ortega responded with an account of the repressive role of the guard in Nicaragua's past, including its formation and training by United States Marines when they occupied Nicaragua in the 1920s and '30s.

At another point, Ortega and contra leader Adolfo Calero discussed the definition of democracy. Calero said the Sandinista would undoubtedly term the contras' variety ``bourgeois.''

Ortega ceded the point, but insisted what the Sandinistas want for Nicaragua is not communist dictatorship, but a ``popular, Nicaraguan democracy. Something new.''

The impression from such reports is that while the Sandinistas have vowed never to discuss internal political issues with the contras, they don't mind talking politics.

``Each side is trying to make the other see its point of view. To explain why they do what they do,'' says one informed source.

Another informed source told reporters in a briefing that the most substantive exchanges seem to be between the military men at the meeting. ``Ortega is so impressive, so lucid, so self-assured that [the contra comandantes] have come to respect him as a man of power,'' said the source.

``These [Sandinistas] are real military men ... Joaqu'in Cuadra [Army chief of staff] is like a tropical Prussian,'' he said. ``These guys ... are serious military men.''

At one point, said the source, Ortega explained to ``Tono'' and ``Fernando'' that when the Sandinistas speak - as they do daily - of the contras as ``Reagan's mercenaries,'' they are not condemning the rebel troops in the field as hired hands.

Rather, he said, the Sandinistas view the policy of the Reagan administration as ``mercenary.'' The military men nodded in agreement.

How far such gestures can be accepted as truly serious, however, must be tempered with the knowledge that the Sandinista government has long condemned the contras as killers of women and children. As when, in early February, the contras set off an antipersonnel mine in northern Nicaragua and killed 19 people riding in a public transport truck.

[According to Reuters, as the two sides entered the third round of talks Wednesday, the state-run Voice of Nicaragua radio reported a contra violation of the truce agreed on for the talks' duration.

[The broadcast said contra troops ambushed an Army truck in the northern province of Matagalpa at 2 p.m. Tuesday, killing two soldiers. Bosco Matamoros, a spokesman for the rebels, told reporters that ``there could be isolated cases, but there is a clear will from both sides to respect that decision (for a truce).'']