Original Receiver
45automan,
From what you have described, the rifle has an orignal GI receiver. If you give us the first few digits of the serial number (e.g., 3,500,xxx - leave off the last three), someone here can tell you the approximate date of manufacture. You can also check serial numbers on the bolt, barrel, and probably one or more other parts, to determine if they are GI, who made them, and when they date from.
As far as this possibly being a "parts gun", you have to define your term. There are very VERY few M-1s left which are composed of all the same parts they left the factory with. Military SOP seems to have been to field strip rifles when necessary, clean them, then usually just grab parts and reassemble the rifles. This inter-changability of parts was a strong point of a battle rifle, but left few examples of a rifle with all of it's original parts intact.
This is not necessarily a bad thing. You can buy a rifle made out of mixed parts, then buy the appropriate parts to make a rifle with at least all parts from one manufacturer and the same time period.
You may be able to get more information from Dick Culver's Shooting page,
http://www.jouster.com
or from Scott Duff's page,
http://www.scott-duff.com
If the rifle seems to be in good shape, if may be worth it. CMP is charging $500 plus &19.95 shipping for service-grade M-1s. Others on this forum, or the two references I gave you can give you some good tips on what to look for.
Good luck! M-1s are good rifles!
Frosty
MOLON LABE