am unclear on how replying to a real OIG\SSA address is an abnormal risk.
i have noticed nothing fishy regarding the origination of the letter...there is nothing to mention on a omsbudsman call regarding 'this letter did not come from OIG\SSA because of typos\barcodes\case#\etc....'

i do not want to risk penalties or suspensions if the letter is ignored. i will reply, via registered mail, that this un-named pension does not exist, so there is no specific information on this pension one can send.
will not be granting permissions of any type, nor sending any documentation.

have dealt with errors from the IRS, it is a huge mistake to speculate what is happening and comment\provide extraneous details in formal communications.
as you say, they should already have access to whatever employer information currently allowed by law, especially 1099-R forms generated from legitimate pensions. so if the latter do exist, i will request OIG\SSA provide it to recipient before further correspondence.

yes, i suspect doge actions, but that will remain speculation.