Mimeo 58, which explicitly excludes donated property from straight-line depreciation, is entwined with the terms letter and forms an integral part of the mutual understanding for the change-over from retirement accounting. See Chicago, Milwaukee, St. Paul Pacific R. R. v. United States, 186 Ct.Cl. 250, 266-272, 287-294, 404 F.2d 960, 969-972 (1968). One of those accepted conditions was the exclusion of donated property.