The county assessor, at least once each year prior to the first Monday in May, shall visit and inspect each place of business, warehouse, factory or other establishment owned and operated by any manufacturer within his county for the purpose of obtaining such information as may be desirable or necessary to provide an accurate basis for comparison with the statement made by such manufacturer under section 150.320. A report of the information so obtained with respect to each manufacturer, in such form as may be prescribed by the county commission, shall be made by the assessor to the county board of equalization.