Maryland Attorney General Francis R. Burch issued an advisory legal opinion today that Gov. Marvin Mandel has the authority to name someone to a House of Delegates seat that has proved difficult to fill.

The vacancy in the seat representing Mandel's old northwest Baltimore district was created by the death in December of veteran Del. Murray Abramson. SInce then, two men have been named to fill the seat, but neither has occupied it.

The first man chosen to replace Abramson, was 22-year-old James Dorf, son of a Baltimore Supreme Bench judge, and grandson of long-time political boss James H. (Jack) Pollack. Dorf was selected by the Democratic Central Committee for his district, with the deciding vote cast by his sister, Jayme, a member of the Central Committee.

Last week the Court of Appeals, the state's highest court, ruled Dorf ineligible because at the time the Central Committee vote was taken, his sister was a resident of Baltimore County, not the city.

Mandel was entitled to make the second selection, since the Central Committee had not made a valid appointment within the 30 days alloted it. He picked 27-year-old Mark I. Frank, an auditor for the state civil defense agency who was completely unknown in political circles before his appointment. Before being sworn in Frank changed his mind about serving in the House, citing job pressures as a reason for turning down the appointment.

Since Frank was never sworn in, said Burch in his opinion, Mandel has the authority to try to find someone else to take Abramson's seat. Had Frank been sworn in, Burch opined, the choice would have rested with the General Committee - which is itself not fully manned since Ms. Dorf has been ruled ineligible.

In other action today, Mandel unveiled his prison master plan for Maryland, which is designed to eliminate the serious overcrowding in state prison facilities by 1982.

Mandel aide Alan Wilner predicted the state will need 9,520 prison beds by January, 1982. Prison rating agencies now rate Maryland's capacity at 5,025 beds, but there are 8,064 prisoners who are the state's responsibility, Wilner said. Of them, 6,957 are in state facilities, and 1,107 in overcrowded local jails.

The master plan hangs on the legislature's approval of state use of an abandoned East Baltimore can factory as a prison. Legislators from East Baltimore have vowed to fight the plan setting up what may be one of this session's biggest legislative battles.