That's an interesting mod, not heard of that....but a logical one for snipers.
I know one of the more popular mods with the Airborne (not necessarily a sniper one) was to remove the elasticated pockets from the parachutist oversmock, and have those re-stitched onto the Denison smock, which allowed extra ammo or grenades etc to be carried, these were often fitted to the lower rear sides of the smocks, or sometimes have been seen on the sleeves as well.