Hey @mrkstn1, thank you so much for the comment. I should have put something in the article about the "HT" stamp. Thank you for pointing that out.

From the research we had done, I was under the impression that methyl bromide was outlawed in every country other than China, so it was not something that would be used on pallets anymore. Here is the information we have found,

[HT] = Heat treatment / [MB] = Methyl Bromide / [DB] = Debarked / [KD] = Kiln Dried

"[MB]: Methyl bromide fumigation, this is a potent pesticide linked to human health problems and ozone layer depletion. Methyl bromide fumigation kills invasive species like pine beetles.

In 1987, the Montreal Protocol aimed to regulate the use of methyl bromide, along with multiple other chemicals found to impact the ozone. This type of treatment is now banned in Canada and many countries because it poses health risks to workers handling the pallets. However, you can still find it in some places. If you find an MB pallet (likely from Asia or Oceania), please do not use it for your craft projects or as firewood, find a waste-removal company that can dispose of it properly."

Source
https://www.1001pallets.com/pallet-safety/amp/

We have personally never seen a pallet marked "MB" but always try to choose our pallets carefully. Our goal in this experiment was to mainly address paints, seals, stains applied to pallets or spills that may have happened in transit.