This manuscript number refers to the original source for the note that he was a prisoner. It was probably a Federal record - a listing of Confederate POWs. As George stated, this manuscript is probably in the National Archives now, BUT, these records are no longer filed by these manuscript numbers that you can find in soldiers' military records. I have spoken with NARA staff about this before, and it would be virtually impossible to find this specific manuscript now, or any other with those numbers that are found on the CSR cards. These numbers were used as a cross-reference by the clerks that compiled the soldiers' service records, from the original records, and there is no such cross-reference in existence today, to know exactly which original record the clerk was using.
This original record MAY even have been microfilmed, because most of the Federal listings of Confederate POWs are on microfilm. But you most likely wouldn't find that specific manuscript number in any index now.
Not much help, I know ....