Before any bill of costs is taxed, the party claiming any item of cost or disbursement shall attach thereto an affidavit, made by himself or by his duly authorized attorney or agent having knowledge of the facts, that such item is correct and has been necessarily incurred in the case and that the services for which fees have been charged were actually and necessarily performed. (June 25, 1948, ch. 646, 62 Stat. 957 .) Historical and Revision Notes Based on title 28, U.S.C., 1940 ed., § 831 (R.S. § 984; June 10, 1921, ch. 18, § 304, 42 Stat. 24 ). Section as revised conforms to existing Federal Practice. See note to subdivision (d) of Rule 54 of the Federal Rules of Civil Procedure. For discussion as to verification of bill of costs under existing practice, see—8 Hughes, Federal Practice, Jurisdiction and Procedure—Civil and Criminal, § 6441. Words “or allowed by the General Accounting Office” were omitted as unnecessary. That office will not allow items in a tax bill for costs against the United States unless such bill has been taxed by the court, and the court, under this section, cannot tax as costs items in an unverified bill. Changes were made in phraseology.