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 cut these five blanks on my fretboard jig a long time ago. Now I finally want one, so I pulled one out at random, and it turned out to be the worst of the batch. By worst, I mean it was thin in the middle and thick at the ends, probably because it is not perfectly flat and and sat on the jig with a slight bow that I didn't notice.