I was curious if anyone knew if it is unusual to find a completely blank planchet? And how would I differentiate between that and a slug?
Well, lets get some terms straight first: A "blank" is the round piece of metal punched from the strip (you may also see this sometimes called a Type I Blank). A "planchet" has been through the upset mill and has raised rims, ready for striking (you may also see this sometimes called a Type II Blank). A "slug" is a garden pest with a low tolerance to salt. Asking about a "blank planchet" introduces confusion, and is redundant. Blanks and Planchets are both fairly common in modern series, and have a fairly low premium for common types (zinc pennies are very common). Finding silver planchets is less common, and will thus have higher premiums.
I've found several blank planchets in bank boxes over the years. Not common. But not out of the realm of possibility.
A type 1 blank will be flat all the way across, with a cut and shear edge. A type 2 blank (planchet) will have a slightly raised face edge, with a smoother circumference edge due to the milling machine. Weight and diameter may help rule out a slug.
This was definitely the best I could ask for! This answered my question perfectly! Thank you for the illustrations.
A "punch out" from an electrical outlet could also be called a "slug" seen a few at a flea mkt trying to be pawned off as a foreign coin blank. Just chuckle to yourself and walk on.
Generally, the electric box knockouts have one or two rough spots leftover from the tab. They are only partly punched out of the box so you knock out the one(s) you need. Leaves a tab. If somebody tries to file it down to make a fake blank coin, check the edge.