A conversion neck is one that can convert an instrument from one scale length to another with no other modification. A common conversion is 34" to 32". By overhanging the 20th fret, or leaving it off, you can use the stock bridge location on the body, just adjust for intonation. Such a neck will work on any standard bass body. A conversion from 34" to 30" requires overhanging two frets, or losing one or both.
I got side-tracked onto this, and at risk of turning this into a furniture thread, I think it's worth sharing. This is just a box. It was motivated by two things:
I often find I am filing something ( like a pickguard ) across the front edge of the workbench - a very awkward angle. I've ruined the natural spot - the right corner - with the vise - worthwhile.
I need a place to store my growing collection of templates, which is getting pretty heavy.