These watermarks were associated with the Royal size (505 x 645 mm, 20" x 25") of a much wider range originally produced for the paper merchants F J Head & Company. (After Mr Head was killed in World War I, his partner Frederick A Brett joined J Barcham Green & Son and we took over the range of papers). The moulds have an FJH monogram and two images, I believe the latter were introduced in the 1930s.
The Head represents that of Christ as transferred to the kerchief of St Berenice (or Veronica as it is said to be the Vera Icon or true image) when she wiped his face as he carried the cross to Golgotha. The date of 1399 is entirely spurious. My Grandfather Jack Green told me he put it in as anyone would know this paper could not have been made in 1399! Actually it could have been as the design idea of the mediaeval laid pattern and watermarks was meant to emulate papers of that period.
The Hand is said to be that of the Pope (although we don't know which one). This is said to be distinguished by the ring on his thumb.
As the photograph of Christ's Head is out of focus, look at moulds M254 which have very similar marks; spot the difference!