That's one theory but I researched this last year and this is what I came up with:
The pheon, or broad arrow, was first used to identify government property by Henry Sidney Earl of Romney (1641-1704), who was Master of Ordnance to King William and Queen Mary from 1693-1702. Henry Sidney was also one of the men who persuaded William of Orange to become King of England.
Henry was instructed to mark all government property in an attempt to reduce theft. He chose to use the Sidney family emblem, which can still be seen in many places at Penshurst (the family seat), and which is still in use by the government today.
Grateful thanks to Robert Noel, Lancaster Herald at the College of Arms London for putting me on the right track.
Eddie