I'm the happe owner of a couple of BM3 ribbon microphones. I inspected the ribbons and after 50 years they could do with a replacement. I found where to buy the 2,5µm alu-foil to make new ribbons. I noticed that the original ribbons are corrugated a little differently than it's commenly shown and illustrated on the web. Instead of corrugating the entire piece, the middle part of the ribbon is left straight. Any comments on that. I wonder if this is a brilliant and important feature or it's just pure coincidence.