On Wed, 3 Feb 2021 at 09:59, Clem Cole <clemc@ccc.com> wrote:

On Wed, Feb 3, 2021 at 5:14 AM Ed Bradford <egbegb2@gmail.com> wrote:
Hay, Arnold,

MC 68K was created in 1980 or thereabouts. We talked about 10's of Megahertz, I think, in those times. 
The original X series part was originally unnumbered but a sticker was later set for the lids that said X68000 (I had one on my desk - which was used for the Tektronix Magnolia prototype).[1]  The X series ran at 8 Mhz, but the original released (distributed - MC68000) part was binned at 8 and 10  as were the later versions with the updated paging microcode called the MC68010 a year later.   When the 68020 was released Moto got the speeds up to 16Mhz and later 20.  By the '040 I think they were running at 50MHz

Was the "X" prefix always used for prototypes?  I remember having an XC68020 in something - might have been an Sun 3/60, or an early Mac IIcx?