I swapped the drum screws for countersunk zinc plated allen key head bolts. As far as I remember, 3/8" - 16 TPI Whitworth is the correct thread size.
3/8 - 16 UNC will work in this case, as not many places stock Whitworth bolts nowadays.
With copperslip on the threads, I have never had problems with them sticking since. With the older drums, the jacking hole was also the same thread size, so you could immediately use the bolt you had just removed to remove the drum.
They seem to have changed the jacking threads to M8x1.25 on the newer drums, which can be a minor pain as it means going to find another bolt to remove the drum.