Production payments, or interests therein evidenced by trust certificates or other instruments, payable from oil, gas or other hydrocarbons in producing properties located in the United States or Canada or the adjacent continental shelf if an obligation secured by and payable from such production payment or interest therein would qualify for investment under § 1308(a) or under § 1308(c)(1) of this title as an obligation which is adequately secured and has investment qualities and characteristics wherein the speculative elements are not predominant. The term “production payments” shall be deemed to mean rights to oil, gas or other hydrocarbons in place or as produced which entitle the owner thereof to a specified fraction or percentage of production until a specified sum of money has been received. No insurer shall make an investment pursuant to this section if the aggregate amount so invested will exceed 15% of its assets.