Vosper 73" ASRL
I have a plan of a 67 foot Thornycroft RAF high speed rescue launch by John Pritchard at a scale of 1/2 inch to the foot. it appears to be similar to the one you mention in the article by John Pritchard, the pennant number on my plan is 2507 but the mast and aerial details are clear and detailed. The plan is from Model Boats Plans Service and was given free with the June 1982 issue.
This may help with your detail.
Dave
Dave