FN produced a pamphlet describing the L9A1 (BHP) pistol they produced for the British Military. Sheet 2 of the pamphlet describes/pictures, among other parts, the "Magazine Assembly/Pistol Automatic 9mm L9A1" and the markings called for by the contract...including the NSN 1005-13-010-0319, the manufacturers symbol - a stylized "FN" and the date of manufacture (DOM).
FNs L9A1 pamphlet has been available...rarely in its original form, reproduced occasionally and is reproduced in various reference books...including the out-of-print "The Browning High Power Automatic Pistol" by Blake Stevens...where there are photos of various L9A1 pistols and IIRC magazines as well. I doubt you'll find the pamphlet itself on the internet. Search for the NSN, it'll at least show up as "Magazine Assembly 9 Millimeter".