In all Residence Districts, any open use involving waste or high-hazard material storage in Use Groups IV(B) or IX, other than a use of land with minor improvements that:

(a)        involves activities not located within a completely enclosed building; and

(b)        involves the use of buildings or other structures or other improvements with a total assessed valuation, excluding land, of less than $20,000 as determined from the assessment rolls effective on the date established for termination;

may be continued for 10 years after December 15, 1961, or such later date that the use becomes non-conforming, provided that after the expiration of that period such non-conforming use shall terminate, and thereafter such land or building or other structure shall be used only for a conforming use.