I suspect it has to do with reducing the crankcase volume.....one way to pack the case a bit more than the rotary valve (of front induction) would allow. This can improve power output if the porting is right.

There may be a weight advantage also. Crankshafts don't need to be as massive when there is no hole to provide strength for.

About the only down sides of a reed set-up is reed float at high rpm and lack of intake timing control. I suppose reeds are noisy so that's another down side for a Pattern application, but that's easier to control. At the rpm we operate, reeds are an excellent, simple solution. And fitting a 3 blader up front helps reduce noise a lot

Choihjin, I get my NordLocks from McMaster Carr. The M5 size comes in two outside diameters....I buy the smaller OD. If you decide to buy from them, you can get the Ultra Copper there too