John Carlson is either a poor Pentax historian or

And the first instant-return reflex mirror in an SLR, among many other firsts. But since JC didn't even know the date of the introduction of the K mount it might be too much to expect him to know of Pentax's earlier history, and of the fact that the m42 screw-mount lenses also work on all Pentax DSLR, although of course they require an adapter.