Insurers issuing the standard form of fire insurance policy provided in Section 4803, Article 48, Title 36, Oklahoma Session Laws 1957, page 401 (36 O.S.Supp.1959, § 4803), are authorized to affix thereto or include therein a written statement that such policy does not cover loss or damage caused by nuclear reaction or nuclear radiation or radioactive contamination, all whether directly or indirectly resulting from an insured peril under said policy; provided, however, that nothing herein contained shall be construed to prohibit the attachment to any such policy of an endorsement or endorsements specifically assuming coverage for loss or damage caused by nuclear reaction, nuclear radiation, or radioactive contamination.