Found a snippet in the "Dating a VM" thread in the armory from '09, however I believe this only applies to the VM &PMI3.
Originally Posted by HP_Lovecraft
First, is the serial 6-digit or 9-digit? The 9-digit ones have a 3-digit batch number added to the front which is the month/year the batch was produced.
As for parts, it can be sketchy as the VM was often upgraded, but a quick rundown might be:
non-ambi w/ long-bolt and camo rail: dates to late 1990.
non-ambi w/ short bolt: dates to 1991
ambi w/ metal plates: dates to 1992
after this they should have batch numbers:
1993- switch to 6-hole valve
1994- switch to larger port
1995- notch in feed port
1996- newer style trigger
1997- internal ec and shorter trigger block
1998- production ends.