Yes, the very strong probability is the stock has been swapped with a rifle stock.  This is common with a variety of Winchester carbines.  I have a M1873 SRC with a rifle buttstock – and that letters.  If it didn’t letter, no one would believe it was originally shipped with a rifle butt.  The seller doesn’t provide the serial number for this carbine, but I suspect it is too late to letter.  It’s also interesting that the seller doesn’t even try to promote this as a special order feature.  

Butt stock condition appears to match the rest of the carbine. May have been swapped but it probably wasn’t recently.

Mike