i'll try to sum up the info concise:
*some source claimed that there was a 5421 with 16bit hicolor support but no 24bit truecolor.
*the cirrus 542x databook v7.0 from may1995 listed that the 5420 supports up to 256color modes, while the 5422 has truecolor support. it has even mentioned an uncommon 5425(5424 plus tvout) that i have never seen, but never mentioned any 5421.
*the cirrus 542x tech ref manual(also from 1995) mentioned no 5421 either, but has some inconsistent statements on the 5420: it listed the 5420 with 16bit ram width and supporting up to 256colors only, but in appendix b5 it described the 5420 rev.b to have truecolor ramdac and 32bit data width.

so, is it possible that the 5421 is a cancelled model that became the 5420 rev.b?