Insurers issuing the standard policy pursuant to section two or any permissible variation thereof are authorized to affix thereto or include therein a written statement that the policy does not cover loss or damage caused by nuclear reaction, nuclear radiation or radioactive contamination, or any combination or all of said causes, whether directly or indirectly resulting from an insured peril under said policy: Provided, however, That nothing therein 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.