You pose a good question for which I don't have a firm answer. All I can tell you is that the later Smith-Corona rifles were fitted with a Sedgley front sight cover. But, I have no idea at what serial number the Sedgley covers started. They may have started at the very beginning for all I know.
The only other option for an early Smith-Corona front sight cover would be one marked with a "G" (no triangle) on the right side and a drawing number on the left side.
Hope this helps.
J.B.