Anything 'Special Ordered' is worth more. Condition has at least as much a bearing on value. After you get your letter go to a Professional Firearms Appraiser and get a written appraisal. If, God forbid, anything happens to the gun you'll need it for an insurance claim.
Do print up a history on the gun, who has owned it and how it was passed down. That can effect value and might keep someone from taking it to a Pawn Shop in the future.