Section 129C. For the satisfactory conduct of a prisoner confined in a prison camp, the commissioner may grant, in addition to the deductions of sentence provided under section one hundred and twenty-nine, a further deduction of sentence of not more than two and one half days for each month while confined in a prison camp. Such further deduction of sentence shall be added to any deduction to which the prisoner is entitled under section one hundred and twenty-nine for computing the minimum term of sentence for release on parole as authorized by section one hundred and thirty-three, or for reducing the term of imprisonment by deduction from the maximum term for which he may be held under his sentence or sentences. A prisoner whose term of imprisonment is reduced shall receive from the commissioner a certificate of discharge on the date which has been determined by such additional deduction from the maximum term of his sentence or sentences.