Not sure how much this helps in identifying the actual chips, but, according to these datasheet sources, Revision C is dated June 1990 and Revision D is dated J

Not sure how much this helps in identifying the actual chips, but, according to these datasheet sources, Revision C is dated June 1990 and Revision D is dated January 1992: https://archive.org/details/bitsavers_amdAm7990A_1770429 https://archive.org/details/bitsavers_amdAm7990A_1770429 (Rev. C) https://www.ardent-tool.com/datasheets/AMD_Am7990.pdf https://www.ardent-tool.com/datasheets/AMD_Am7990.pdf (Rev. D)