Appears to be a W on the right bolt lug end and has SU under the left lug.
Ebay seller is tagmil. Appears to have quite the potty mouth if you read his negative and neutral feedback replies to buyers.

The Auction: WW2 US M-1 carbine Winchester M1 Carbine complete Flat type Bolt W on lug | eBay

SU Underwood Marking Under Left Lug:



W On Right Lug:



Different Bolt To Compare A Real WRA W Stamp:

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