Ah, yes. 5mm. That's the easiest and best way to convert from QR to non-QR. Unless you want to spring for locking skewers like Pitlocks. Though I did actually swap out QR axles for solid track axles in a pair of Campy road hubs many years ago, before I'd ever heard of allen skewers (perhaps they didn't exist yet?). That was actually pretty easy to do at the time too since Campy's road and track hubs then had the same threading and same OLD.

An advantage of skewered axles, either QR or non-QR, that I don't think has been mentioned yet is that they can make it easier to adjust the rear wheel position on a bike with horizontal dropouts, especially on a hub gear or single-speed with no derailler or chain tensioner, since the skewer tightens both sides simultaneously. There aren't many hub gears that can use a skewer though, Rohloff is the only one I can think of.