Mr. Hoffman was a very busy builder and apart from the X-1, he created at least 4 other single-engined aircraft, including:
X-2 "Little Orphan Annie", two-seat flying boat, 1962, N9159R c/n 1
X-3 "The Girl Friend", single-seat landplane, 1976, N14647 c/n 1
X-4 "Mullet Skif", two-seat amphibian, 1980, N5597N c/n 1
X-5 "Seabird", single-seat amphibian, 1991, N7182P c/n 1.
I had the pleasure of meeting Mr. Hoffman and his wife many years ago during a visit to Holland. As far as I know, the X-1, X-3, X-4, and X-5 are kept in excellent and airworthy condition by Mr. Hoffman's son."