Internal audit scrutiny: ledger-based recording and explanation thresholds now govern officer mistakes and reporting procedures. Commissioners must maintain ledger cards for each Income-tax Officer recording department-accepted mistakes with material tax effect; cards transfer with the officer and inform character-roll considerations without unduly influencing overall annual assessment. Explanations are mandatory for cases appearing as draft paragraphs from external audit and otherwise at Commissioners' discretion after assessing mistake circumstances; explanations must always be obtained where negligence, impropriety or doubt about bona fides exists. For draft-paragraph cases, explanations or their gists with Commissioner observations must be sent to the Board. Quarterly arrears statement requirements are cancelled due to the ledger system.