Upon every person engaging or continuing in the business of banking, small loan or other financial business, the tax shall be equal to one percent (1%) of the gross income received from interest, premiums, discounts, dividends, service fees or charges, commissions, fines, rents from real or tangible personal property, however denominated, royalties, charges for bookkeeping or data processing, receipts from check sales and the sale of tangible personal property; provided, however, that gross income shall not include interest received on the obligations of the United States, States, or any political sub-division of any of the foregoing, or any of the agencies or instrumentalities of the foregoing, nor shall it include interest received on investments or loans primarily secured by first mortgages or deeds of trust on residential property.