Here is a LMC with Retained Struck Through Thread. Small bit of white thread on the E of united. Coin has 5 areas of struck through on reverse.
I think your coin may be an intrinsic metallic inclusion. I say this due to there being two separate spots of color. Check here. https://www.error-ref.com/improper-alloy-mix/ Point of order: the nickel you posted is not a Retained Struck Through Coin as stated in the thread tittle..