cards. This royalty is due and payable 30 days from date of shipment ..."
 "11. In the event of no sale or production of cards the minimum retail price of the cards shall be figured at 15 cts."
 On April 19, 1946, paragraph 9 of this contract was amended to read as follows:
 "9. The royalty to be paid Sass-Dorne Studio by the California Greeting Card Company shall be 10% of the gross billing for the greeting cards. This royalty is due and payable 30 days from date of shipment." (Italics added.)
 Forty-six of such negatives were delivered to defendants on the date of the writing. Five months later the contract was amended by substituting four words, to wit, "the gross billing for" in lieu of "the retailselling price" in paragraph 9. Otherwise there was no change.
 In January, 1950, plaintiffs sued on the contract in three counts, (1) for 10 per cent of the gross billing of the guaranteed minimum production of 15,000 greeting cards manufactured from each of the 46 plates and sold during the period *210 ending November 19, 1947, in the sum of $10,300; (2) for the same amount as 10 per cent of the gross billing of the guaranteed minimum production of 15,000 cards manufactured from each plate in the next two years, alleging that the contract was renewed automatically by reason of the failure of defendants to give written notice of its cancellation 60 days prior to November 20, 1947; (3) for possession of plates, color separations and original