(a) There may be a provision as follows:
"Insurance with Other Insurers: If there be other valid coverage, not with this insurer, providing benefits for the same loss on other than an expense-incurred basis and of which this insurer has not been given written notice prior to the occurrence or commencement of loss, the only liability for such benefits under this policy shall be for such proportion of the indemnities otherwise provided hereunder for such loss as the like indemnities of which the insurer had notice (including the indemnities under this policy) bear to the total amount of all like indemnities for such loss and for the return of such portion of the premium paid as shall exceed the pro rata portion for the indemnities thus determined."
(b) If the foregoing policy provision is included in a policy which also contains the policy provision set out in Section 27-19-20, there shall be added to the caption of the foregoing provision the phrase "- Other Benefits." The insurer may, at its option, include in this provision a definition of "other valid coverage," approved as to form by the commissioner, which definition shall be limited in subject matter to coverage provided by organizations subject to regulation by insurance law or by insurance authorities of this, or any other, state of the United States or any province of Canada and to any other coverage the inclusion of which may be approved by the commissioner. In the absence of such definition, such term shall not include group insurance or benefits provided by union welfare plans, or by employer or employee benefit organizations. For the purpose of applying the foregoing policy provision with respect to any insured, any amount of benefit provided for such insured pursuant to any compulsory benefit statute, including any workmen's compensation or employer's liability statute, whether provided by a governmental agency or otherwise, shall, in all cases, be deemed to be "other valid coverage" of which the insurer has had notice. In applying the foregoing policy provision, no third party liability coverage shall be included as "other valid coverage."