Personally, I took thebigsilly's response about MOD to be a joke, and I thought it was kinda funny.
To be persnickety, if mod. is an abbreviation for "modification," then it needs a period after it. Maybe it's an acronym for "Modification Of Design," in which case "MOD" is correct. Although, commonly used acronyms often drop the capitals, as in radar and laser. In which case, "mod" is correct.
