In Illinois prisons, one cadre of inmates has been behind bars so long that even their identification numbers are outdated.

There are between 170 and 230 “C-number” prisoners in the state, sent to serve a range of years instead of a finite term before the state changed its rules in 1978.

The name derives from the letter that precedes their inmate ID numbers, which is no longer used for newer inmates. Many of the prisoners were convicted of crimes such as rape or murder.

But as states across the country are looking for ways to reshape their prison systems and cut costs, Illinois’ longest-serving inmates could benefit from the newest reforms.

A panel of lawmakers last year recommended the General Assembly take a hard look at the “effects of incarceration of elderly, ill prisoners who no longer pose a threat to society” in order to help tame the $1.3 billion annual prisons budget.

Releasing C-number prisoners could offer significant savings, as prison watchdog group the John Howard Association estimates each one costs about $70,000 a year to incarcerate.

Additionally, a 2010 state law required prison officials in 2013 to begin using a risk-assessment tool — a set of criteria to help determine the best rehab services for inmates and their likelihood of offending again — to evaluate the prison population.

But that time period overlapped with a significant state budget crunch, and corrections officials last summer told legislators tasked with examining prison reforms that they were still working on implementing risk assessment.

Still, advocates for the C-number prisoners last week filed a lawsuit to compel the Department of Corrections to use the tactic, saying the board that reviews their cases is violating the law by not using standard risk assessments in deciding whether the inmates should get parole.

The suit “doesn’t require anybody to reach a particular conclusion,” said David M. Shapiro, clinical assistant professor of law at Northwestern University School of Law’s MacArthur Justice Center and one of the lawyers who filed the suit.

“It just says, ‘The law says you have to consider this, so consider it.’ ”

Filed in Cook County Circuit Court, the suit alleges the prisons department has “flouted” the law, and without using objective risk assessments, parole decisions for the C-number prisoners have been made arbitrarily.

A spokesman for the Illinois Department of Corrections declined to comment on the lawsuit.

The lead plaintiff, Harrison Chancy, was convicted in 1977 for an armed robbery and murder in Lemont. He was 19 years old at the time.

He has mostly maintained his innocence throughout his 37 years in prison. His lone divergence from that stance was a false admission, said one of his attorneys, Joshua A. Tepfer — a Northwestern clinical assistant professor of law — intended to show remorse and bolster his chances of being paroled and seeing his ill mother.

“Innocent people, people proven innocent by DNA … have falsely admitted their guilt, so it’s not a unique situation,” Tepfer said.

In the meantime, changes could be on the horizon for the 15-member Prisoner Review Board that makes parole decisions for the C-number population.

Several of the members’ terms are technically expired, and they’re acting in the interim until Gov. Bruce Rauner either reappoints them or appoints new individuals.

Aviva Futorian, an attorney who trains other lawyers to handle C-number cases and advocate for such inmates, said those members have until about mid-March until their interim authority expires.

“And, of course, we’re trying like crazy to get some good people appointed,” she said.