as far as serial numbers are concerned, go to
www.jouster.com.
There are about 10 parts on a Garand that are labeled by manufacturer: rear sight, gas plug, reciever, bolt, charging rod, trigger assembley, etc. (I can't remember the others off hand). International Harvester put serial numbers in the stocks.
One that is completely original is worth the most. Ones that have been rebuilt with parts from just one manufacturer (including the right barrel date for the serial number) come in second. Ones with mixed and matched parts follow.
The reason for so many CMP Garands to have mismatched parts is that the Army armorers would simply strip down a defective rifle and toss all of its parts into like piles. Because they were interchangable, rifles were rebuilt with all different parts. Hope that helps some.
Mark