Straits Settlements 1 Cent Coin - Incused Lettering Error?

Brockage error also did come to my mind first.

Is this a PMJ, trying to create a mirror brockage error by hammering original coin into a blank planchet? Because the thickness is the same as original.

And about the reverse, only small portion at 4 o'clock are incused. Possible to do it post mint?

Given the amount of pitting and other damage this coin has, I think it has suffered from a nasty case of bronze disease, where the high points like the tops of the letters were infected, but the low points were not. When the corrosion was removed, the entire letter disappeared, leaving a letter-shaped crater behind.

A similar thing happened to me a few years ago with an old Australian copper token I cleaned the bronze disease off of. I posted my experiences back in this old thread.

Before:


After:


On this token, the words "AND SON" in the centre of the coin, and most of the letters TBU in "Westbury", have "become incuse". I suspect the OP's coin saw a similar effect, only worse.