On October 4 , 1967,"NASA informed the Boeing Company that it was terminating procurement of 10 S-IC stages, S-IC-16 through S-IC-25. Boeing had submitted a proposal on July 3 and an adjusted proposal on September 20 covering the necessary supplies and services for fabrication and delivery of the stages."
-MAF, Historical Report, Jan. 1-Dec. 31, 1967, p. 28.
These were for the second production run of the Saturn V, possibly corresponding to Apollo 21-30 and later two were planned to launch Voyager 1 and 2. The long lead items for S-IC-16 and S-IC-17 had already started when the decision was announced and any materials already gathered were scrapped. Although NASA concluded to lobby for funding for SA-516 and SA-517 (along with Saturn IBs SA-215 and SA-216) until August 1968.
Note: photos are not of S-IC-16 and S-IC-17.
NASA Doc: link
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