His formal name, Avon B. Chisholm, appears near the tail end of Acting Gov. Blair Lee's list of executive aides, tucked in under the heading "Greenbelt Office/Special Assignments."

The vagueness of his job description is appropriate, for after nine years in the State House, he remains one of those political phantoms whose exact duties seem unclear even to those closest to him - including Lee himself.

Nobody much cared when Lee was just lieutenant governor and "Chiz," as he asks people to call him, was just a pleasant enough man seen darting in and out of the State House, always wearing his perpetually flipped-up green sunshades. But Lee became acting governor last year with the trial and conviction of Maryland Gov. Marvin Mandel and the question is cropping up again, what does he do?

Lee has difficulty providing a clear answer.

He's supposed to stay in the Greenbelt office and do casework," Lee said, "but it does seem hard to keep him down on the farm . . . He does all kinds of odd jobs. He keeps me supplied with bills and calendars and things like that.He really is the most industrious kind of guy."

Other staff members describe him as "a logistical expert" who works "hellacious hours" but always ends up being "a thorn in everybody's side."

"I'm the staff garbage collector," said Chisholm when asked to describe his duties. "I look at myself as the caretaker of the whole system here."

Whatever he does, Chisholm puts some 1,000 miles a week on his car - a 1973 Mercedes equipped with a radio telephone and two-way radios. He is constantly going to meetings of some sort all over the state, from Somerset County on the Eastern Shore to his own town of Hyattstown in northern Montgomery County to Garrett County in far Western Maryland.

Then he comes back to Annapolis, to run around from one committee hearing to the next, always with his sunshades flipped up, like a green awning over his eyes, always wearing the PT-109 tieclasp that he got "when I worked for President Kennedy," and always wearing an expression of knowledgeability.

Nobody really knows what he did for the Kennedy administration. ("I worked with media relations and photography," Chiz says vaguely.) Nobody really knows what he does at the committee hearings. ("I'm called on to explain fire and rescue issues," he says.)

But eventually everybody finds out something about what Chiz does the hard say - after he's done it. They find out because some powerful politician or other, perhaps some legislator whose good will they have been sultivating for months, is screaming at them to "keep Chiz the hell out of the way."

"There are two words for Chiz," Acting Gov. Lee said recently, shaking his head with a bemused smile "Industrious and undiplomatic. He keeps barging into delicate questions like a bull in a china shop . . .

To understand why this imperturbale, 5-foot-9, 41-year-old mystery man is constantly setting off some political brush fire it is necessary to understand that he spends most of his time dealing with some of the most combustible, if obscure, issues in Maryland politics - those involving volunteer firefighters.

There are some 30,000 volunteer firefighters in the state, and they wield considerable political power, particularly in the rural areas where the fire house is the political and social nucleus of the community.

Since Chisholm first joined the volunteer fire department in Silver Spring when he was 16, in 1953, he has immersed himself in the world of firefighters and constantly inserted himself into its labyrinthine politics.

Within the past two months, for instance, he has been able to infuriate Prince George's County Fire Chief Frank Bruigulio - and, by extension, County Executive Winfield Kelly - by taking it upon himself to insist that there whould be a new ambulance at Park station, contrary to the wishes of the College Park fire fighters and everybody else involved.

There was another recent explosion when Chiz got into the middle of a dispute about training regulations for new fire fighters also involved in the dispute was the Southern Maryland Firemen's Association and it wasn't long before Speaker of the House John Hanson Briscoe - who comes from rural St. Mary's County - got angry and complained.

The complaints roll right off Chiz's back, though. "Sometimes you have to make a decision and there will always be people who don't like your decision," he said with equanimity.

The authority he has to made decisions, however, is in question. As he explained it, "Wherever I go, and no matter what I do, it's always in the governor's name because I work for him. This jobs carries very heavy responsibilities."

Most of the people he meets, however, can't seem to figure out exactly what those responsibilities are. Chiz himself seems to cultivate that vagueness as he shuttle between the world of the firemen and the world of the politicians.

Those worlds seem to be Chisholm's only two great loves, though he has done an incredible potpourri of other things. He did some dairy farming as a boy when he lived with his parents in Massachusetts. He worked as a postal inspector in the early 1960s. He held and still holds and interest in companies that make communications equipment like two-way radios and he did forensic photography for a while - taking on assigments from private detectives who wanted evidence that would help their clients in divorce cases.

No matter what the jobs was, however, Chisholm (who is paid $24,451) always indicates that his work was tied into the important events of the time. When he was a postal inspector, he said "I worked on the great Boston mail robbery."

Lee hired him in 1968 to fill a part-time job as "a glorified messenger," when Lee was still a state senator. When Gov. Marvin Mandel called on Lee to become secretary of state, and then lieutenant governor, I felt obligated not to just ditch him," Lee said.

Why does Lee keep him on? "I guess I'm just softhearted," the governor said, "but he usually has a pretty good answer for most of the complaints. It's not so much what he does as the way he does it."

Chisholm sees his role as crucial. "Someday somebody's going to come in here after me and do the same kind of work, going to continue on where I left off."

"A sobering thought," Lee said when he heard that.