Knowing that the regiment was in the vicinity of Warrenton at the time, 10 miles below Vicksburg, I queried a CW board if there was a Confederate cemetery at Warrenton. I received an answer rather shortly thereafter as negative, but that I should contact the Old Court House Museum.
This I did, Gordon Cotton told me of the recent find by Jeff Giambrone and that Issac was on the list, but shown as in the 48th Tennessee. An obvious transposition. The rest is history.
Re your ordering of T527, this is what the National Archives says of the 'T' series publications: "Unlike 'M' publications [they] do not usually reproduce a complete series of records; that is, they may contain only segments, by date or subject, of a larger series. In many cases, 'T' publications were produced in response to specific reference requests."
Before you order you might want to check to see if this microfilm has what you want. If you know someone in D. C., perhaps you could ask them to take a look. I've lost my contacts. Also, you might want to call the "Old Army" archivists at the NA to see if they can be of help.
There maybe someone monitoring this board who could assist.
Cheers,
George