The 300 nautical miles zone:
The cover shown above went by ship (Vigilant) to St. Thomas from which it received the 4 rings cancels and backstamp St. Thomas March 9 1903. From St. It was then shipped to Castries St. Lucia at
which it received the backstamp Castries St. Lucia March 12.
The distance between St Thomas and Castries St. Lucia is approx 636 km which equals 343 Nautical Miles.
The distance between Christiansted to Castries St. Lucia is approx. 575 km which
is 311 nautical miles.
The sender was technically correct in franking the cover 8 cents as the actual distance exceeded the 300 nautical miles zone by 11 nautical miles – but the St. Thomas Post Office did not cancel the bisect as they accepted
the distance between Christiansted – Castries St. Lucia to be within the 300 nautical miles from Christiansted and the postage to be 5 cents accordingly.