How to tell speed of an unknown Z80 ?

I need a 3mhz Z80 to fix a game board.

I've got some old random boards (not games, I think I pulled them from a
dedicated terminal years ago) with Z80s on them. Any idea how to tell what
speed they are from the numbers ? They are Mostek MK3880N. There is also a 4
digit number next to the other info -- one has 8026, one has 8135

==========================================================
Chris Candreva -- chris@westnet.com -- (914) 967-7816
WestNet Internet Services of Westchester
http://www.westnet.com/
Received on Wed Sep 29 19:38:00 1999