Section 21B. Evidence of the reputation of a victim's sexual conduct shall not be admissible in an investigation or proceeding before a grand jury or a court of the commonwealth for a violation of sections 13B, 13B1/2, 13B3/4, 13F, 13H, 22, 22A, 22B, 22C, 23, 23A, 23B, 24, 24B, 50 or 51 of chapter 265 Evidence of specific instances of a victim's sexual conduct in such an investigation or proceeding shall not be admissible except evidence of the victim's sexual conduct with the defendant or evidence of recent conduct of the victim alleged to be the cause of any physical feature, characteristic, or condition of the victim; provided, however, that such evidence shall be admissible only after an in camera hearing on a written motion for admission of same and an offer of proof. If, after said hearing, the court finds that the weight and relevancy of said evidence is sufficient to outweigh its prejudicial effect to the victim, the evidence shall be admitted; otherwise not. If the proceeding is a trial with jury, said hearing shall be held in the absence of the jury. The finding of the court shall be in writing and filed but shall not be made available to the jury.