In the diagram, split pins are used to form locating eyes in the lower surface of the upper wing on bi-planes, also they are used as eyes in the tops of the inter-plane struts. The struts are permanently fixed to the lower wing by interlocking the split pin eyes like a chain link before fitting one in the wing & the other in the bottom of the strut.
To fit the struts to the upper wing it is the age old method of slipping the pin through the eye of the front strut, then through the front eye & rear eye on the underside of the wing, then through the rear strut eye and then stretch an elastic band around over the front end of the pin and around over the back end of the pin.
This method is also effectively used for holding the lower end of wing struts to the lower fuselage on high wing models like our own Corben Baby Ace