It's a bit confusing as it's an old English things were they would use both of and off together i.e. She picked it up off of the floor.
The term off as used in picking lists or numbers of parts to produced is better understood if you think about going to a stores and asking for a number of parts to be taken "off" the shelf or "removed"
So when you ask for 12 off washers. You would be asking for 12 to be removed from the box of washers..
At the end of the day it's just a term used/derived from a manufacturing back ground.
The term off as used in picking lists or numbers of parts to produced is better understood if you think about going to a stores and asking for a number of parts to be taken "off" the shelf or "removed"
So when you ask for 12 off washers. You would be asking for 12 to be removed from the box of washers..
At the end of the day it's just a term used/derived from a manufacturing back ground.