M598 may be the same thing we refer to as NARA Tape 40, but I don't know that for sure. We don't have access to ancestry.com but other genealogy researchers have provided information from time to time citing documents they found at ancestry.com. Some have found records of packages received and money being kept by the Fort Delaware prison staff on the account of various POWs. So long as you are looking at an image of the actual document, I consider the information valid. I do not put much faith in ancestry's third party transcripts of the Compiled Military Service Records prefering to look at an image of the actual CMSR cards. I understand that these CMSR cards are transcripts as well, but at least the War Department clerks who did the work were military records professionals.

Glad to be of help! Keep me posted!

Hugh