I used to get my flanges from Weirtech, they are laser cut. With regard to the slots into the bolt holes, it was mentioned before and I agree, that these may just be because it's quicker to cut the slot than repierce the material. I ran a waterjet at my last job and it would take much longer to pierce 3/8 steel than cut two 1/4 inch slots. Plus piercing material makes an absolute mess. Next time I get more flanges made I'll ask my local waterjet job shop if it'll be cheaper to make slots into each hole.

 

Damn these things are pretty. When I have money again look out for an order from me.