What are the differences between the 82-259 and 82-239?

Hello! Thanks for your question. The difference between these two mortises is a matter of materials used in their make. The 82-259 is a stainless steel part, whereas 82-239 is made out of zinc-plated steel.

The 82-259 and 82-239 are otherwise identical. Generally, you want to use the stainless steel version if you live in a salt-air environment, or just if corrosion is an issue you're concerned about. Otherwise the 82-239 is perfectly applicable in all the same applciations.