Source: Distillery,

The first bourbon in our wood-finishing series, Maker's Mark 46 was created by Bill Samuels, Jr., to amplify the flavors he loves in Maker's Mark. The innovative wood-stave-finishing process starts with fully matured Maker's Mark at cask strength. We then insert 10 seared virgin French oak staves into the barrel and finish it for nine weeks in our limestone cellar. The result is Maker's Mark 46: bolder and more complex, but without the bitterness typical of longer-aged whiskies.

So, why Maker's Mark 46?
The wood-finishing experiments were largely enacted by Bill Samuels, Jr., and Master Distiller Kevin Smith, with a big assist from the Independent Stave Company, the distillery's longtime cooperage. After finally finding the flavor they were after, Bill, Jr., learned that the perfectly toasted French oak stave was on file as "Stave Profile No. 46."