The buttplate is Smith Corona, probably original to the rifle.
The trigger guard is from an early '03. You can tell by the fact that it has been drilled for a pin in front of the trigger slot. Is the pin in place? More info on that will follow.
Can you show a photo of the top of the action, showing the bolt assembly?
The "K" in the mag cut-off well is for Keystone. They made C-stocks during WWII to be used as replacements for A3's. They were also original on some M1903A4 sniper rifles. If it looks a little dry you can apply some BLO(boiled linseed oil
) to liven it up.