How were the prods fastened down? I put a big washer over the center-hole (as recommended by the manufacturer) then I screwed the prod down with a 5/16 in. lag-screw about 3 inches long. Over that, I fitted a 3/4 in thick by 4 inch wide bridle (binding) block, (recessed in the center to accomodate the bolt) and lashed the prod down with flax cord, much as I would do an un-bolted prod. This was in the early days, so I probably didn't get the cord as tight as I do nowadays. When the binding was tight and complete, I painted the bridle-cord with hide-glue to help immobilize it. Prods broke anyhow (after several years of shooting) when they broke, they actually pulled the lag-bolt out of the stock. On the positive side, there was no separation of prod-ends, they didn't go flying and nobody was hurt, but as the guy who made them, I was surprised. After it happened several times over the years, I got used to it. Solution: I replaced the customer's prods with new, un-drilled prods and lashed 'em down in the usual way.
My assumption is, the prods were still flexing slightly in the center, and the hole was the weakest point. After a few years of shooting, the binding-cord probably loosened up a bit, leaving the center-bolt to do most of the work. In retrospect, I wonder how many Jayhawk kit crossbows broke through the center? Not many, I think, since Jayhawk provided a steel string with swaged loops on the ends. After a hundred or so shots, the string would stretch enough to fly off at every shot. The ends of the prod were slotted, with sharp edges, so if you tried to make a fiber-string, it would cut in no time. Result... probably most people who made a Jayhawk kit gave up trying after a few weeks and the bows ended up in a garage-sale to snag some new sucker. Still the aluminum-alloy prods were what put me in business. When the old couple who owned the company retired, I bought some aluminum and started making my own. If anybody's interested, the alloy is 7075-T6. I use 190 thousandths of an inch... that's 3/16 in. And yes, the metal eventually fatigues and fails, but they're great little prods while they work. Geezer.