After waiting 3 months my CMP M1 Special finally arrived along with a few cases of Greek surplus M2 ball. The Springfield receiver serial is in the 5,9XX,XXX range.
Here's a thread on the subject:
http://forums.thecmp.org/showthread.php?t=41117
This is a short, helpful guide with photos:
http://www.trfindley.com/pgtgfix.html