Hi Tom,
That certainly looks like an original Jeckells sail from around 1970-1976 (right down to the stitching).
The luff wire is also the norm for any SeaHawk headsail, and I believe for any English boat other than the lightest dinghies (but I am not an expert on those).
The "W" is a mystery, as is the "H" mentioned on another US boat and also seen on just one of the last boats to be built by Mistral Craft. See:
http://www.seahawk17.org.uk/gallery/uk/isolde.htmIf you are very lucky that you might be able to read the job number on the sail. It should be somewhere near the Jeckells sail logo and will be stamped in indelible black ink on the sail. Jeckells do claim that they have full records of all sails despatched from their loft since at least the early 1970s. If you can quote that number at them they may be able to help with details of the original order.