<i>> my guess, combined with the fact that “70” is clearly stamped on after the fact, is that these are TMM2064P chips that have been tested and certified to work at even faster speeds like 70ns. Generally the fastest RAM you need on a board like this is for the linebuffer, so that’d be my guess here.</i><p>Overlooking a small detail there: the fastest RAM on that board would be the 2 chips marked MCM2016HN45 (the -45 means 45ns for that part, which is pretty fast for a RAM chip made in 1986).<p>Anyway, very nice & detailed writeup!