Is this just a new stall tactic? Has anyone else got this letter when working to repair their credit?
This is a new one for me! I would write back (with copies of original request), and affirm the information is from you, and that you insist upon compliance with the FCRA.
They do it every now and then. Yes, it is a ploy to get you to send something additional in so that it will extend the deadline to 45 days. Don't because they conduce the re-investigations anyway.
You could always send a summons on the basis of their failure to conducted even the cursory investigation required by FDCPA - and since the original stuff you sent was on a letter over your signature and the signature on the Summons will also be yours, the Judge is likely to rule that the CRA violated FCRA.