That's the Mk I version. The Mk II was a development in a slightly different direction - it had a thread, but no screwdriver recess. The Mk III combined the best features of Mk I and MK II, in that it had no thread and no screwdriver recess, and was thus very cheap to manufacture.
Whilst this development work was ongoing, a competitor launched a version with both screwdriver recess AND thread, cornered the market, and drove the first company out of business.
Thus, the original Mk I, Mk II and Mk III versions are very rare, and will in due course command high prices from collectors.
(P.S. If you are struggling to get a screw in, try a smear of Vaseline. It will slip in much easier.)