I confess ignorance about such devices. As far as I know they were never standard equipment on Reedcraft boats. My father's, bought new, certainly did not have one.
I was always taught that if you wanted a kicking strap when "roller-reefed" (which is the default method on a SeaHawk) you simply roll in a length of strap with a shackle on the end and attach the kicker to that.
Personally, I have never used one. I'm quite happy to let the boom lift and spill wind when reefed. After all, to some extent spilling wind is what reefing is all about, but I dare say that ex-racers always want maximum efficiency. Perhaps I'm missing something?
Further, I had assumed that Harry's device would have had a way of opening the "horse-shoe" to get it over the boom and than locking it again. But I see no mechanism for that. How, exactly, do you use it?
You see lots of these at boat jumbles, probably because they are pretty useless. I had one and used only once. You feed it onto the sail from the topping lift end but I found it more trouble than help. It also threatened to abrade my sail. As Greg says, sail shape isn't that important when reefed down using roller reefing, although a few turns of line around the furled sail then attached to the kicking strap does help the sail to drive better, and it's kinder to the sail. I dumped roller reefing on my boat and now use conventional slab reefing which gives a much better sail shape.