2 minths SPP-II-B (30years) arrear bill sent to D.T.O for pre audit. Is it correct?

YES — It is correct. For SPP-II-B (30 years) cases:
This is a higher grade / pay fixation benefit.
Any arrear bill related to pay revision, FR 22, SPP, SPP-I, SPP-II-A, SPP-II-B must undergo PRE-AUDIT.
Pre-audit is done only at District Treasury Office, not at Sub-Treasury.
So the Sub-Treasury Officer (STO) must send your arrear bill to the District Treasury Office (DTO) for verification and sanction.
This is the correct and mandatory procedure in AP Treasury Rules.