It does seem odd - I would not have though that a great number survived when records suggest that only 200,000 of the steel version were manufactured.
The 'Flanders Flap' were manufactured during 1915 by J. Purdey & Sons, A. Purdey, M. Myers & Son and Brandauer & Co.
Prices varied from 2 1/2d per 100 (or about 1p today, these were made from canvas) for 10,000 units from A. Purdey. 1/3d (6p) per 100 for 50,000 units of basic protectors from J. Purdey & Sons.
The sheet metal version War Office purchases were /|\ marked and bore the contractors name, these cost 10/- (50p) per 100 on 100,000 ordered from Brandauer and 8/-(40p) per 100 on 100,000 ordered from Myers.
The correct nomenclature was I believe "Protector, muzzle, No.3 Mark 1"