I'll let the 100-Six gurus conjure up the cut-off point/date/chassis number if they have it, but my understanding has always been that only early 100-Sixes came with the ridged bonnet. Because some people just like their looks, they have installed them on later cars.
In Clausager's 1990 book, Original Austin-Healey, he states, on page 65: "Some 100-Six cars had a central crease in the bonnet bulge, never found on the 3000." Given the level of detail of that book and the lists of production changes in it, I'm slightly surprised that he wasn't more definitive, but perhaps it was just one of those running changes and no one bothered to record the chassis number where it first occurred.
(P.S. Donald Healey didn't make Austin-Healeys, apart from the 100S and the first 20 or so prototype BN1s; BMC made them.)