I would go Service Grade - you'll be getting more original parts.
Not necessarily. Except for Correct and Collector's grade, the grade of the rifle is relative to condition of the rifle, not an indication of what parts have been installed in the rifle. A Service grade is in better condition than a Field grade that is better than a Rack grade.
A huge percentage of Garands have been through at least one rebuild, either in the field or at an arsenal, many have gone through multiple rebuilds over time particularly WWII vintage rifles. Since parts of the Garand are not serial numbered, the ones removed from a rifle during rebuild do not necessarily get returned to that rifle.
Also, one of the "holy grails" of Garand collectors is finding parts made during the appropriate time the receiver was originally made. Parts are marked with the drawing number and revision number that dates the approximate time the part was manufactured.
But, if you read some of books by Scott Duff, Bruce Canfield and others, you'll find frequently parts that were installed during the original assembly of a rifle that were made outside the date range of the receiver or even made by one of the other manufacturers. It all depended on what parts were on hand at the time and what was needed to meet production quotas.
Rifle's sent by CMP are somewhat governed by the luck of the draw.
If you are mail ordering a Garand, I'd go with a Service grade, it will be in better condition than a Field grade or Rack grade. Service grades are excellent rifles, but many are disappointed with the condition of the rifle when they receive it.
If you want a Garand that looks new, go with a CMP Special grade. Unfortunately, they are out of stock at present.
