Natdigger, just for clarification, I was asking about the ridges which run "vertically" down from the sabot's top to its bottom. One of them shows in your first photo at the tip of your finger. Are there only two of those? Or are you talking about ridges which encircle the sabot at its edges?
Because Natdigger says it seems to have three faint lands-&-grooves rifling marks, it may have been fired from a CS 3.4" Reamed-&-Rifled 4-pounder Smoothbore. I say "may" because that cannon's rifling grooves are the same width as its lands. (Lands and grooves are each approximately 1.75" wide.) Is that what you see on this sabot, Natdigger?
Regards,
Pete