I now own several different leg straps for the scabbard of the M3 knife (either M6 of M8 types). The shortest one in this picture is a replica for certain. The two other ones are supposed to be authentic. They sure look authentic, in that they look old and used, but I have some questions. First of all, all three straps are of different length and the two at the bottom look like they originally used to be attached to something else. Secondly, they are being offered on eBay at pretty low prices. Is someone making new straps from original parts? Thirdly: were these straps ever official issue items at all? Or were they removed from other items? When you carefully look at WWII photographs and books on the subject, you will often see trench knives attached to the legs by another type of strap altogether: a sort of white strap with a larger buckle, but it doesn’t seem to be a tourniquet.
More pictures…

Many of you reacted to this post (albeit by e-mail, not in the comments), and the main conclusion is that there’s no such thing as an official issue leg strap, and soldiers would use whatever they could get their hands on. Don Burgett (veteran) says he and his buddies cut straps off musette bags and adapted them to that purpose. Mark Bando says he also has a photo of a 501 man wearing the wide shoulder strap from a musette bag, looped through the upper part of a M-8 scabbard and wrapped around the upper ankle. If you actually try this using an M-8, you will discover that the wider strap lends more stability and doesn’t slide up and down like a narrow web strap. Also, the D loops on each end can be clipped to the wire prongs at the top of the scabbard, usually used for affixing the scabbard to the pistol or cartridge belt. With a little stretching pressure, the D rings can be hooked to those wire loops and no other fastening devises are required. Of course a regular M-8 scabbard doesn’t have the wire hanging devices but some modified ones do, and all M-8 A-1 scabbards have them as well. Johan Willaert also tells me that all sorts of straps were used, even tourniquets. What I saw in many D-Day period photographs of paratroopers would be the white linen straps from the field cots on which they had slept on the days before 6 June 1944. I should get a picture of that.
As for the straps in my pictures, Ken Lewis informs me that they were removed from the pack-tail from the M-1928 pack. It has a strap at the top top and bottom and you will see that the buckle is the same. Some straps that are being sold and used are USMC straps from the field pack for attaching the tent or camo poncho to the pack in a horse shoe shape. Most of these have a different buckle and a square tip at the other end, also on the end where the buckle is, the small return of strap holding the buckle is waxed, which is common to Marine webbing items.
Thank you all!