I suppose there is the possibilty exists, since the Belgian M51 was a French gas mask, that initially the French supplied them complete with a bag (which is what you have), and then as time went by, the Belgians placed contracts for their own version of the Bag, and started to buy just the mask from the French?
This could possibly explain why Belgian tongue and loop fasteners are found on the vast majority of Belgian M51 bags. Initially the Belgians copied a French supplied type fairly closel, using a "Lift The Dot" type of fastener, and later moved to a tongue and loop to standardise the Gas mask bag with the closures used elsewhere on Belgian 37 Pattern inspired ammunition pouches etc?
My sole LTD gas mask bag is undated so I cannot offer any evidence that the LTD bag were made before the Tongue and Loop bags, though.
Your LTD Belgian bag also seems to be undated/ unstamped.