Mirror Class rules allow a varitey of kicking strap systems. The kicking strap supplied with the boat in the 1960's was very crude - just a hemp line with an eye splice that went around the boom and a deck clip (lacing eye) on the centreline of the stowage bulkhead where the line was fastened by a knot. Since then kicking straps have become easier to use and more powerful. Current class rules allow up 8:1 purchase on a conventional kicking strap or a strut system (normally called a GNAV which is VANG spelt backwards) mounted above the boom.
The original practice of having the kicking strap attached to the boom by a loop around it carries on. It's simple and mechanically very strong. The strain on a fitting on top of the boom to locate the attachment is minimal. On the other hand if you put a fitting, such as a deck clip, on the bottom surface of the boom and attach the kicking strap to that, the fastenings will be under considerable tension. Mechancally this is quite weak.
The location of the kicking strap attachment point on the boom used to be controlled in the class rules. When centre mainsheet systems were introduced this rule was removed to enable the kicking strap to be located further forward and give more room for the crew. The location of the kicking strap is now "self policing" since moving the attachment point forward reduces the length of the lever formed by the boom pivoting at the gooseneck and this makes the kicking strap less effective. The rule used to be for the kicking strap to be attached a minimum of 483mm from the inner end of the boom. On my own boats I currently have the kicking strap attached 433mm from the inner end of the boom.
The other end of the kicking strap can be attached to the stowage bulkhead or to the bottom of the mast. Since the Bermudan rig was introduced there has been a trend in the UK to attach the kicking strap to the bottom of the mast. This location means more thrust is directed along the line of the boom, helping to bend the mast. It also gives more room for the crew.
Below I will add links to descriptions of the most common kicking strap & GNAV systems dreckly.