I have put the 2 knives together and now I'm not sure if it's a 15. Compare the distance between the bolts and notches, and also from the bolt to the edge of the hilt. Schirras seems to be a greater distance, an indication of an Astro hilt.
Also in the comparison, Schirras knife is more at an angle to the camera, whereas the 15 is flat. This angle difference makes Schirras points of distance appear shorter than they actually are.
Here it is with Slaytons handle. Hard to tell.