Look on the top rear of the receiver to find the manufacturer. Sometimes these may be hard to see as they can be partially obstructed by the receiver sight. No original carbines came with a metal upper handguard, although it is conceivable this might be a replacement for a cracked handguard.
If this is a genuine GI carbine, $400 is a very low price. So, either the seller is uninformed, or as other posters have suggested, it may be a commercially manufactured copy (e.g., Universal, Plainfield, etc.). If that is the case, you should know that these are generally believed to be questionably reliable and inferior to the originals. Personally I would not pay $400 for one of those.