No proof shall be satisfactory, within the meaning of RCW 81.16.010 through 81.16.030, unless it includes the original (or verified copies) of the relevant cost records and other relevant accounts of the affiliated interest, or such abstract thereof or summary taken therefrom, as the commission may deem adequate, properly identified and duly authenticated: PROVIDED, HOWEVER, That the commission may, where reasonable, approve or disapprove such contracts or arrangements without the submission of such cost records or accounts.