No Date Lincoln Memorial Cent Mint Error - Split After Strike 1.44g

Super cool! I've never seen a split planchet on a memorial that new.

Nice pickup.
I wonder if that small curve migh have been caused by being struck by another cent while still in the collar (if your coin didn't eject)? There appears to be an incuse rim present along the outer edge of the anomaly. The reverse doesn't seem damaged like it would be if this curve happened outside the collar.

The uniform fin on the obverse, while unusual, seems possible on a clamshell split planchet (from metal flow).
CoinWorld article on Clamshell splits- https://www.coinworld.com/news/prec...-are-un.html

The smaller Rim Fin on the reverse reminded me that lots of times a Rim Fin is associated with a tilted die strike or a localized increase in striking pressure.
"Definition: A fin is a thin flange that extends vertically from the rim/edge junction of a coin struck within the collar. It is caused by excessive striking pressure or a localized increase in striking pressure caused by slight die tilt. The extra pressure causes coin metal to squeeze into the narrow gap between die neck and collar. Fins can develop on either face or both faces simultaneously. A fin may encircle a coin or may be restricted to one pole. The latter is the kind typically produced by die tilt." (underlining and bold highlighting is by me for emphasis.)
Rim Fin link:
http://www.error-ref.com/finning/

Is the small extra curve opposite the O of ONE?

It sounds like your coin got stuck in the collar and was struck by a second coin along the edge of the rim.
The second coin may have severed the clamshell at the edge of this second coin and ejected the missing part of the clamshell from the rest of the coin.

Everyone feel free to knock some sense into my imagination.

However it happened makes for a great Halloween mystery.

The thickness of the fin is the height of the split off. Nice find. Unusual looking that way it split off. But I've seen this happen on another coin that they had both halves of.

Thanks TB. (BTW- Yours is a 1973 looking at the designers initials- unique single year reverse design with the large, high initials.)
https://www.PCGS.com/coinfacts/coin...3-1c-rd/2965
So the incuse curve is not directly opposite but close to the highest fin area.
Maybe this drawing will show what I think may have happened better than I can describe it.



It seems possible the force of the second strike may have caused a weakly bonded clamshell to separate.

The 1973 P, D, or S & 1.7 gram mate to yours is out there somewhere with an indented curve (unless it became part of a capped die.) I don't imagine you'll keep an eye out for it.
The curved dent would be just to the left of Lincolns bust.