General NA citation question (subgroups)

I understand that subgroups should be a part of citations to records in the National Archives. I've noticed that records in the National Archives Catalog (in the infoormation sidebar) list the Record Group, Series, and File Unit, but do not give you the Subgroup. Am I missing this somewhere? Also, I look through the guide and found the list of finding aids pertaining to Preliminary Inventories, but I have been unable to locate the actual PIs themselves.

For example, this one...

Finding Aids: Evelyn Wade, comp., "Preliminary Inventory of Bureau of Pensions Correspondence and Pension and Bounty-Land Case Files Relating to Military Service Performed Between 1775 and 1861," NM 22 (1964).

I have had no luck locating this online or anywhere for that matter. Am I missing a simple way to locate the subgroup for a given series? I would think that information, if truly pertinent to a proper citation would be easy to find. Funny that the record is easier to locate that the info for the citation. :)

Your thoughts would be much appreciated as always.